Re: [PATCH -tip 23/32] sched: Add a per-thread core scheduling interface

[chris hyser]

On 12/14/20 6:25 PM, Joel Fernandes wrote:
> On Mon, Dec 14, 2020 at 02:44:09PM -0500, chris hyser wrote:
> > On 12/14/20 2:31 PM, Joel Fernandes wrote:
> > > > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> > > > index cffdfab..50c31f3 100644
> > > > --- a/kernel/sched/debug.c
> > > > +++ b/kernel/sched/debug.c
> > > > @@ -1030,6 +1030,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
> > > >    #ifdef CONFIG_SCHED_CORE
> > > >    	__PS("core_cookie", p->core_cookie);
> > > > +	__PS("core_task_cookie", p->core_task_cookie);
> > > >    #endif
> > >
> > > Hmm, so the final cookie of the task is always p->core_cookie. This is what
> > > the scheduler uses. All other fields are ingredients to derive the final
> > > cookie value.
> > >
> > > I will drop this hunk from your overall diff, but let me know if you
> > > disagree!
> >
> >
> > No problem. That was there primarily for debugging.
>
> Ok. I squashed Josh's changes into this patch and several of my fixups. So
> there'll be 3 patches:
> 1. CGroup + prctl  (single patch as it is hell to split it)
> 2. Documentation
> 3. ksefltests
>
> Below is the diff of #1. I still have to squash in the stop_machine removal
> and some more review changes. But other than that, please take a look and let
> me know anything that's odd.  I will test further as well.
>
> Also next series will only be interface as I want to see if I can get lucky
> enough to have Peter look at it before he leaves for PTO next week.
> For the other features, I will post different series as I prepare them. One
> series for interface, and another for kernel protection / migration changes.
>
> ---8<-----------------------
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index a60868165590..73baca11d743 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -688,6 +688,8 @@ struct task_struct {
>   #ifdef CONFIG_SCHED_CORE
>   	struct rb_node			core_node;
>   	unsigned long			core_cookie;
> +	unsigned long			core_task_cookie;
> +	unsigned long			core_group_cookie;
>   	unsigned int			core_occupation;
>   #endif
>   
> @@ -2081,11 +2083,15 @@ void sched_core_unsafe_enter(void);
>   void sched_core_unsafe_exit(void);
>   bool sched_core_wait_till_safe(unsigned long ti_check);
>   bool sched_core_kernel_protected(void);
> +int sched_core_share_pid(unsigned long flags, pid_t pid);
> +void sched_tsk_free(struct task_struct *tsk);
>   #else
>   #define sched_core_unsafe_enter(ignore) do { } while (0)
>   #define sched_core_unsafe_exit(ignore) do { } while (0)
>   #define sched_core_wait_till_safe(ignore) do { } while (0)
>   #define sched_core_kernel_protected(ignore) do { } while (0)
> +#define sched_core_share_pid(flags, pid) do { } while (0)
> +#define sched_tsk_free(tsk) do { } while (0)
>   #endif
>   
>   #endif
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index c334e6a02e5f..3752006842e1 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -248,4 +248,10 @@ struct prctl_mm_map {
>   #define PR_SET_IO_FLUSHER		57
>   #define PR_GET_IO_FLUSHER		58
>   
> +/* Request the scheduler to share a core */
> +#define PR_SCHED_CORE_SHARE		59
> +#define PR_SCHED_CORE_CLEAR		0  /* clear core_sched cookie of pid */
> +#define PR_SCHED_CORE_SHARE_FROM	1  /* get core_sched cookie from pid */
> +#define PR_SCHED_CORE_SHARE_TO		2  /* push core_sched cookie to pid */
> +
>   #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 7199d359690c..5468c93829c5 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -736,6 +736,7 @@ void __put_task_struct(struct task_struct *tsk)
>   	exit_creds(tsk);
>   	delayacct_tsk_free(tsk);
>   	put_signal_struct(tsk->signal);
> +	sched_tsk_free(tsk);
>   
>   	if (!profile_handoff_task(tsk))
>   		free_task(tsk);
> diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
> index 5fc9c9b70862..c526c20adf9d 100644
> --- a/kernel/sched/Makefile
> +++ b/kernel/sched/Makefile
> @@ -36,3 +36,4 @@ obj-$(CONFIG_CPU_FREQ_GOV_SCHEDUTIL) += cpufreq_schedutil.o
>   obj-$(CONFIG_MEMBARRIER) += membarrier.o
>   obj-$(CONFIG_CPU_ISOLATION) += isolation.o
>   obj-$(CONFIG_PSI) += psi.o
> +obj-$(CONFIG_SCHED_CORE) += coretag.o
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7f807a84cc30..80daca9c5930 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -157,7 +157,33 @@ static inline bool __sched_core_less(struct task_struct *a, struct task_struct *
>   	return false;
>   }
>   
> -static void sched_core_enqueue(struct rq *rq, struct task_struct *p)
> +static bool sched_core_empty(struct rq *rq)
> +{
> +	return RB_EMPTY_ROOT(&rq->core_tree);
> +}
> +
> +static struct task_struct *sched_core_first(struct rq *rq)
> +{
> +	struct task_struct *task;
> +
> +	task = container_of(rb_first(&rq->core_tree), struct task_struct, core_node);
> +	return task;
> +}
> +
> +static void sched_core_flush(int cpu)
> +{
> +	struct rq *rq = cpu_rq(cpu);
> +	struct task_struct *task;
> +
> +	while (!sched_core_empty(rq)) {
> +		task = sched_core_first(rq);
> +		rb_erase(&task->core_node, &rq->core_tree);
> +		RB_CLEAR_NODE(&task->core_node);
> +	}
> +	rq->core->core_task_seq++;
> +}
> +
> +void sched_core_enqueue(struct rq *rq, struct task_struct *p)
>   {
>   	struct rb_node *parent, **node;
>   	struct task_struct *node_task;
> @@ -184,14 +210,15 @@ static void sched_core_enqueue(struct rq *rq, struct task_struct *p)
>   	rb_insert_color(&p->core_node, &rq->core_tree);
>   }
>   
> -static void sched_core_dequeue(struct rq *rq, struct task_struct *p)
> +void sched_core_dequeue(struct rq *rq, struct task_struct *p)
>   {
>   	rq->core->core_task_seq++;
>   
> -	if (!p->core_cookie)
> +	if (!sched_core_enqueued(p))
>   		return;
>   
>   	rb_erase(&p->core_node, &rq->core_tree);
> +	RB_CLEAR_NODE(&p->core_node);
>   }
>   
>   /*
> @@ -255,8 +282,24 @@ static int __sched_core_stopper(void *data)
>   	bool enabled = !!(unsigned long)data;
>   	int cpu;
>   
> -	for_each_possible_cpu(cpu)
> -		cpu_rq(cpu)->core_enabled = enabled;
> +	for_each_possible_cpu(cpu) {
> +		struct rq *rq = cpu_rq(cpu);
> +
> +		WARN_ON_ONCE(enabled == rq->core_enabled);
> +
> +		if (!enabled || (enabled && cpumask_weight(cpu_smt_mask(cpu)) >= 2)) {
> +			/*
> +			 * All active and migrating tasks will have already
> +			 * been removed from core queue when we clear the
> +			 * cgroup tags. However, dying tasks could still be
> +			 * left in core queue. Flush them here.
> +			 */
> +			if (!enabled)
> +				sched_core_flush(cpu);
> +
> +			rq->core_enabled = enabled;
> +		}
> +	}
>   
>   	return 0;
>   }
> @@ -266,7 +309,11 @@ static int sched_core_count;
>   
>   static void __sched_core_enable(void)
>   {
> -	// XXX verify there are no cookie tasks (yet)
> +	int cpu;
> +
> +	/* verify there are no cookie tasks (yet) */
> +	for_each_online_cpu(cpu)
> +		BUG_ON(!sched_core_empty(cpu_rq(cpu)));
>   
>   	static_branch_enable(&__sched_core_enabled);
>   	stop_machine(__sched_core_stopper, (void *)true, NULL);
> @@ -274,8 +321,6 @@ static void __sched_core_enable(void)
>   
>   static void __sched_core_disable(void)
>   {
> -	// XXX verify there are no cookie tasks (left)
> -
>   	stop_machine(__sched_core_stopper, (void *)false, NULL);
>   	static_branch_disable(&__sched_core_enabled);
>   }
> @@ -295,12 +340,6 @@ void sched_core_put(void)
>   		__sched_core_disable();
>   	mutex_unlock(&sched_core_mutex);
>   }
> -
> -#else /* !CONFIG_SCHED_CORE */
> -
> -static inline void sched_core_enqueue(struct rq *rq, struct task_struct *p) { }
> -static inline void sched_core_dequeue(struct rq *rq, struct task_struct *p) { }
> -
>   #endif /* CONFIG_SCHED_CORE */
>   
>   /*
> @@ -3779,6 +3818,9 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
>   	p->capture_control = NULL;
>   #endif
>   	init_numa_balancing(clone_flags, p);
> +#ifdef CONFIG_SCHED_CORE
> +	p->core_task_cookie = 0;
> +#endif
>   #ifdef CONFIG_SMP
>   	p->wake_entry.u_flags = CSD_TYPE_TTWU;
>   	p->migration_pending = NULL;
> @@ -3903,6 +3945,7 @@ static inline void init_schedstats(void) {}
>   int sched_fork(unsigned long clone_flags, struct task_struct *p)
>   {
>   	unsigned long flags;
> +	int __maybe_unused ret;
>   
>   	__sched_fork(clone_flags, p);
>   	/*
> @@ -3978,6 +4021,13 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
>   #ifdef CONFIG_SMP
>   	plist_node_init(&p->pushable_tasks, MAX_PRIO);
>   	RB_CLEAR_NODE(&p->pushable_dl_tasks);
> +#endif
> +#ifdef CONFIG_SCHED_CORE
> +	RB_CLEAR_NODE(&p->core_node);
> +
> +	ret = sched_core_fork(p, clone_flags);
> +	if (ret)
> +		return ret;
>   #endif
>   	return 0;
>   }
> @@ -7979,6 +8029,9 @@ void init_idle(struct task_struct *idle, int cpu)
>   #ifdef CONFIG_SMP
>   	sprintf(idle->comm, "%s/%d", INIT_TASK_COMM, cpu);
>   #endif
> +#ifdef CONFIG_SCHED_CORE
> +	RB_CLEAR_NODE(&idle->core_node);
> +#endif
>   }
>   
>   #ifdef CONFIG_SMP
> @@ -8983,6 +9036,9 @@ void sched_offline_group(struct task_group *tg)
>   	spin_unlock_irqrestore(&task_group_lock, flags);
>   }
>   
> +void cpu_core_get_group_cookie(struct task_group *tg,
> +			       unsigned long *group_cookie_ptr);
> +
>   static void sched_change_group(struct task_struct *tsk, int type)
>   {
>   	struct task_group *tg;
> @@ -8995,6 +9051,11 @@ static void sched_change_group(struct task_struct *tsk, int type)
>   	tg = container_of(task_css_check(tsk, cpu_cgrp_id, true),
>   			  struct task_group, css);
>   	tg = autogroup_task_group(tsk, tg);
> +
> +#ifdef CONFIG_SCHED_CORE
> +	sched_core_change_group(tsk, tg);
> +#endif
> +
>   	tsk->sched_task_group = tg;
>   
>   #ifdef CONFIG_FAIR_GROUP_SCHED
> @@ -9047,11 +9108,6 @@ void sched_move_task(struct task_struct *tsk)
>   	task_rq_unlock(rq, tsk, &rf);
>   }
>   
> -static inline struct task_group *css_tg(struct cgroup_subsys_state *css)
> -{
> -	return css ? container_of(css, struct task_group, css) : NULL;
> -}
> -
>   static struct cgroup_subsys_state *
>   cpu_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
>   {
> @@ -9087,6 +9143,18 @@ static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
>   	return 0;
>   }
>   
> +static void cpu_cgroup_css_offline(struct cgroup_subsys_state *css)
> +{
> +#ifdef CONFIG_SCHED_CORE
> +	struct task_group *tg = css_tg(css);
> +
> +	if (tg->core_tagged) {
> +		sched_core_put();
> +		tg->core_tagged = 0;
> +	}
> +#endif
> +}
> +
>   static void cpu_cgroup_css_released(struct cgroup_subsys_state *css)
>   {
>   	struct task_group *tg = css_tg(css);
> @@ -9688,6 +9756,22 @@ static struct cftype cpu_legacy_files[] = {
>   		.write_u64 = cpu_rt_period_write_uint,
>   	},
>   #endif
> +#ifdef CONFIG_SCHED_CORE
> +	{
> +		.name = "core_tag",
> +		.flags = CFTYPE_NOT_ON_ROOT,
> +		.read_u64 = cpu_core_tag_read_u64,
> +		.write_u64 = cpu_core_tag_write_u64,
> +	},
> +#ifdef CONFIG_SCHED_DEBUG
> +	/* Read the group cookie. */
> +	{
> +		.name = "core_group_cookie",
> +		.flags = CFTYPE_NOT_ON_ROOT,
> +		.read_u64 = cpu_core_group_cookie_read_u64,
> +	},
> +#endif
> +#endif
>   #ifdef CONFIG_UCLAMP_TASK_GROUP
>   	{
>   		.name = "uclamp.min",
> @@ -9861,6 +9945,22 @@ static struct cftype cpu_files[] = {
>   		.write_s64 = cpu_weight_nice_write_s64,
>   	},
>   #endif
> +#ifdef CONFIG_SCHED_CORE
> +	{
> +		.name = "core_tag",
> +		.flags = CFTYPE_NOT_ON_ROOT,
> +		.read_u64 = cpu_core_tag_read_u64,
> +		.write_u64 = cpu_core_tag_write_u64,
> +	},
> +#ifdef CONFIG_SCHED_DEBUG
> +	/* Read the group cookie. */
> +	{
> +		.name = "core_group_cookie",
> +		.flags = CFTYPE_NOT_ON_ROOT,
> +		.read_u64 = cpu_core_group_cookie_read_u64,
> +	},
> +#endif
> +#endif
>   #ifdef CONFIG_CFS_BANDWIDTH
>   	{
>   		.name = "max",
> @@ -9889,6 +9989,7 @@ static struct cftype cpu_files[] = {
>   struct cgroup_subsys cpu_cgrp_subsys = {
>   	.css_alloc	= cpu_cgroup_css_alloc,
>   	.css_online	= cpu_cgroup_css_online,
> +	.css_offline	= cpu_cgroup_css_offline,
>   	.css_released	= cpu_cgroup_css_released,
>   	.css_free	= cpu_cgroup_css_free,
>   	.css_extra_stat_show = cpu_extra_stat_show,
> diff --git a/kernel/sched/coretag.c b/kernel/sched/coretag.c
> new file mode 100644
> index 000000000000..4eeb956382ee
> --- /dev/null
> +++ b/kernel/sched/coretag.c
> @@ -0,0 +1,734 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * kernel/sched/core-tag.c
> + *
> + * Core-scheduling tagging interface support.
> + *
> + * Copyright(C) 2020, Joel Fernandes.
> + * Initial interfacing code  by Peter Ziljstra.
> + */
> +
> +#include <linux/prctl.h>
> +#include "sched.h"
> +
> +/*
> + * Wrapper representing a complete cookie. The address of the cookie is used as
> + * a unique identifier. Each cookie has a unique permutation of the internal
> + * cookie fields.
> + */
> +struct sched_core_cookie {
> +	unsigned long task_cookie;
> +	unsigned long group_cookie;
> +
> +	struct rb_node node;
> +	refcount_t refcnt;
> +};
> +
> +/*
> + * A simple wrapper around refcount. An allocated sched_core_task_cookie's
> + * address is used to compute the cookie of the task.
> + */
> +struct sched_core_task_cookie {
> +	refcount_t refcnt;
> +	struct work_struct work; /* to free in WQ context. */;
> +};
> +
> +static DEFINE_MUTEX(sched_core_tasks_mutex);
> +
> +/* All active sched_core_cookies */
> +static struct rb_root sched_core_cookies = RB_ROOT;
> +static DEFINE_RAW_SPINLOCK(sched_core_cookies_lock);
> +
> +/*
> + * Returns the following:
> + * a < b  => -1
> + * a == b => 0
> + * a > b  => 1
> + */
> +static int sched_core_cookie_cmp(const struct sched_core_cookie *a,
> +				 const struct sched_core_cookie *b)
> +{
> +#define COOKIE_CMP_RETURN(field) do {		\
> +	if (a->field < b->field)		\
> +		return -1;			\
> +	else if (a->field > b->field)		\
> +		return 1;			\
> +} while (0)					\
> +
> +	COOKIE_CMP_RETURN(task_cookie);
> +	COOKIE_CMP_RETURN(group_cookie);
> +
> +	/* all cookie fields match */
> +	return 0;
> +
> +#undef COOKIE_CMP_RETURN
> +}
> +
> +static inline void __sched_core_erase_cookie(struct sched_core_cookie *cookie)
> +{
> +	lockdep_assert_held(&sched_core_cookies_lock);
> +
> +	/* Already removed */
> +	if (RB_EMPTY_NODE(&cookie->node))
> +		return;
> +
> +	rb_erase(&cookie->node, &sched_core_cookies);
> +	RB_CLEAR_NODE(&cookie->node);
> +}
> +
> +/* Called when a task no longer points to the cookie in question */
> +static void sched_core_put_cookie(struct sched_core_cookie *cookie)
> +{
> +	unsigned long flags;
> +
> +	if (!cookie)
> +		return;
> +
> +	if (refcount_dec_and_test(&cookie->refcnt)) {
> +		raw_spin_lock_irqsave(&sched_core_cookies_lock, flags);
> +		__sched_core_erase_cookie(cookie);
> +		raw_spin_unlock_irqrestore(&sched_core_cookies_lock, flags);
> +		kfree(cookie);
> +	}
> +}
> +
> +/*
> + * A task's core cookie is a compound structure composed of various cookie
> + * fields (task_cookie, group_cookie). The overall core_cookie is
> + * a pointer to a struct containing those values. This function either finds
> + * an existing core_cookie or creates a new one, and then updates the task's
> + * core_cookie to point to it. Additionally, it handles the necessary reference
> + * counting.
> + *
> + * REQUIRES: task_rq(p) lock or called from cpu_stopper.
> + * Doing so ensures that we do not cause races/corruption by modifying/reading
> + * task cookie fields.
> + */
> +static void __sched_core_update_cookie(struct task_struct *p)
> +{
> +	struct rb_node *parent, **node;
> +	struct sched_core_cookie *node_core_cookie, *match;
> +	static const struct sched_core_cookie zero_cookie;
> +	struct sched_core_cookie temp = {
> +		.task_cookie	= p->core_task_cookie,
> +		.group_cookie	= p->core_group_cookie,
> +	};
> +	const bool is_zero_cookie =
> +		(sched_core_cookie_cmp(&temp, &zero_cookie) == 0);
> +	struct sched_core_cookie *const curr_cookie =
> +		(struct sched_core_cookie *)p->core_cookie;
> +	unsigned long flags;
> +
> +	/*
> +	 * Already have a cookie matching the requested settings? Nothing to
> +	 * do.
> +	 */
> +	if ((curr_cookie && sched_core_cookie_cmp(curr_cookie, &temp) == 0) ||
> +	    (!curr_cookie && is_zero_cookie))
> +		return;
> +
> +	raw_spin_lock_irqsave(&sched_core_cookies_lock, flags);
> +
> +	if (is_zero_cookie) {
> +		match = NULL;
> +		goto finish;
> +	}
> +
> +retry:
> +	match = NULL;
> +
> +	node = &sched_core_cookies.rb_node;
> +	parent = *node;
> +	while (*node) {
> +		int cmp;
> +
> +		node_core_cookie =
> +			container_of(*node, struct sched_core_cookie, node);
> +		parent = *node;
> +
> +		cmp = sched_core_cookie_cmp(&temp, node_core_cookie);
> +		if (cmp < 0) {
> +			node = &parent->rb_left;
> +		} else if (cmp > 0) {
> +			node = &parent->rb_right;
> +		} else {
> +			match = node_core_cookie;
> +			break;
> +		}
> +	}
> +
> +	if (!match) {
> +		/* No existing cookie; create and insert one */
> +		match = kmalloc(sizeof(struct sched_core_cookie), GFP_ATOMIC);
> +
> +		/* Fall back to zero cookie */
> +		if (WARN_ON_ONCE(!match))
> +			goto finish;
> +
> +		match->task_cookie = temp.task_cookie;
> +		match->group_cookie = temp.group_cookie;
> +		refcount_set(&match->refcnt, 1);
> +
> +		rb_link_node(&match->node, parent, node);
> +		rb_insert_color(&match->node, &sched_core_cookies);
> +	} else {
> +		/*
> +		 * Cookie exists, increment refcnt. If refcnt is currently 0,
> +		 * we're racing with a put() (refcnt decremented but cookie not
> +		 * yet removed from the tree). In this case, we can simply
> +		 * perform the removal ourselves and retry.
> +		 * sched_core_put_cookie() will still function correctly.
> +		 */
> +		if (unlikely(!refcount_inc_not_zero(&match->refcnt))) {
> +			__sched_core_erase_cookie(match);
> +			goto retry;
> +		}
> +	}
> +
> +finish:
> +	/*
> +	 * Set the core_cookie under the cookies lock. This guarantees that
> +	 * p->core_cookie cannot be freed while the cookies lock is held in
> +	 * sched_core_fork().
> +	 */
> +	p->core_cookie = (unsigned long)match;
> +
> +	raw_spin_unlock_irqrestore(&sched_core_cookies_lock, flags);
> +
> +	sched_core_put_cookie(curr_cookie);
> +}
> +
> +/*
> + * sched_core_update_cookie - Common helper to update a task's core cookie. This
> + * updates the selected cookie field and then updates the overall cookie.
> + * @p: The task whose cookie should be updated.
> + * @cookie: The new cookie.
> + * @cookie_type: The cookie field to which the cookie corresponds.
> + *
> + * REQUIRES: either task_rq(p)->lock held or called from a stop-machine handler.
> + * Doing so ensures that we do not cause races/corruption by modifying/reading
> + * task cookie fields.
> + */
> +static void sched_core_update_cookie(struct task_struct *p, unsigned long cookie,
> +				     enum sched_core_cookie_type cookie_type)
> +{
> +	if (!p)
> +		return;
> +
> +	switch (cookie_type) {
> +	case sched_core_no_update:
> +		break;
> +	case sched_core_task_cookie_type:
> +		p->core_task_cookie = cookie;
> +		break;
> +	case sched_core_group_cookie_type:
> +		p->core_group_cookie = cookie;
> +		break;
> +	default:
> +		WARN_ON_ONCE(1);
> +	}
> +
> +	/* Set p->core_cookie, which is the overall cookie */
> +	__sched_core_update_cookie(p);
> +
> +	if (sched_core_enqueued(p)) {
> +		sched_core_dequeue(task_rq(p), p);
> +		if (!p->core_cookie)
> +			return;
> +	}
> +
> +	if (sched_core_enabled(task_rq(p)) &&
> +	    p->core_cookie && task_on_rq_queued(p))
> +		sched_core_enqueue(task_rq(p), p);
> +}
> +
> +#ifdef CONFIG_CGROUP_SCHED
> +void cpu_core_get_group_cookie(struct task_group *tg,
> +			       unsigned long *group_cookie_ptr);
> +
> +void sched_core_change_group(struct task_struct *p, struct task_group *new_tg)
> +{
> +	unsigned long new_group_cookie;
> +
> +	cpu_core_get_group_cookie(new_tg, &new_group_cookie);
> +
> +	if (p->core_group_cookie == new_group_cookie)
> +		return;
> +
> +	p->core_group_cookie = new_group_cookie;
> +
> +	__sched_core_update_cookie(p);
> +}
> +#endif
> +
> +/* Per-task interface: Used by fork(2) and prctl(2). */
> +static void sched_core_put_cookie_work(struct work_struct *ws);
> +
> +/* Caller has to call sched_core_get() if non-zero value is returned. */
> +static unsigned long sched_core_alloc_task_cookie(void)
> +{
> +	struct sched_core_task_cookie *ck =
> +		kmalloc(sizeof(struct sched_core_task_cookie), GFP_KERNEL);
> +
> +	if (!ck)
> +		return 0;
> +	refcount_set(&ck->refcnt, 1);
> +	INIT_WORK(&ck->work, sched_core_put_cookie_work);
> +
> +	return (unsigned long)ck;
> +}
> +
> +static void sched_core_get_task_cookie(unsigned long cookie)
> +{
> +	struct sched_core_task_cookie *ptr =
> +		(struct sched_core_task_cookie *)cookie;
> +
> +	refcount_inc(&ptr->refcnt);
> +}
> +
> +static void sched_core_put_task_cookie(unsigned long cookie)
> +{
> +	struct sched_core_task_cookie *ptr =
> +		(struct sched_core_task_cookie *)cookie;
> +
> +	if (refcount_dec_and_test(&ptr->refcnt))
> +		kfree(ptr);
> +}
> +
> +static void sched_core_put_cookie_work(struct work_struct *ws)
> +{
> +	struct sched_core_task_cookie *ck =
> +		container_of(ws, struct sched_core_task_cookie, work);
> +
> +	sched_core_put_task_cookie((unsigned long)ck);
> +	sched_core_put();
> +}
> +
> +struct sched_core_task_write_tag {
> +	struct task_struct *tasks[2];
> +	unsigned long cookies[2];
> +};
> +
> +/*
> + * Ensure that the task has been requeued. The stopper ensures that the task cannot
> + * be migrated to a different CPU while its core scheduler queue state is being updated.
> + * It also makes sure to requeue a task if it was running actively on another CPU.
> + */
> +static int sched_core_task_join_stopper(void *data)
> +{
> +	struct sched_core_task_write_tag *tag = (struct sched_core_task_write_tag *)data;
> +	int i;
> +
> +	for (i = 0; i < 2; i++)
> +		sched_core_update_cookie(tag->tasks[i], tag->cookies[i],
> +					 sched_core_task_cookie_type);
> +
> +	return 0;
> +}
> +
> +int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)
> +{
> +	struct sched_core_task_write_tag wr = {}; /* for stop machine. */
> +	bool sched_core_put_after_stopper = false;
> +	unsigned long cookie;
> +	int ret = -ENOMEM;
> +
> +	mutex_lock(&sched_core_tasks_mutex);
> +
> +	if (!t2) {
> +		if (t1->core_task_cookie) {
> +			sched_core_put_task_cookie(t1->core_task_cookie);
> +			sched_core_put_after_stopper = true;
> +			wr.tasks[0] = t1; /* Keep wr.cookies[0] reset for t1. */
> +		}
> +	} else if (t1 == t2) {
> +		/* Assign a unique per-task cookie solely for t1. */
> +
> +		cookie = sched_core_alloc_task_cookie();
> +		if (!cookie)
> +			goto out_unlock;
> +		sched_core_get();
> +
> +		if (t1->core_task_cookie) {
> +			sched_core_put_task_cookie(t1->core_task_cookie);
> +			sched_core_put_after_stopper = true;
> +		}
> +		wr.tasks[0] = t1;
> +		wr.cookies[0] = cookie;
> +	} else if (!t1->core_task_cookie && !t2->core_task_cookie) {
> +		/*
> +		 * 		t1		joining		t2
> +		 * CASE 1:
> +		 * before	0				0
> +		 * after	new cookie			new cookie
> +		 *
> +		 * CASE 2:
> +		 * before	X (non-zero)			0
> +		 * after	0				0
> +		 *
> +		 * CASE 3:
> +		 * before	0				X (non-zero)
> +		 * after	X				X
> +		 *
> +		 * CASE 4:
> +		 * before	Y (non-zero)			X (non-zero)
> +		 * after	X				X
> +		 */
> +
> +		/* CASE 1. */
> +		cookie = sched_core_alloc_task_cookie();
> +		if (!cookie)
> +			goto out_unlock;
> +		sched_core_get(); /* For the alloc. */
> +
> +		/* Add another reference for the other task. */
> +		sched_core_get_task_cookie(cookie);
> +		sched_core_get(); /* For the other task. */
> +
> +		wr.tasks[0] = t1;
> +		wr.tasks[1] = t2;
> +		wr.cookies[0] = wr.cookies[1] = cookie;
> +
> +	} else if (t1->core_task_cookie && !t2->core_task_cookie) {
> +		/* CASE 2. */
> +		sched_core_put_task_cookie(t1->core_task_cookie);
> +		sched_core_put_after_stopper = true;
> +
> +		wr.tasks[0] = t1; /* Reset cookie for t1. */
> +
> +	} else if (!t1->core_task_cookie && t2->core_task_cookie) {
> +		/* CASE 3. */
> +		sched_core_get_task_cookie(t2->core_task_cookie);
> +		sched_core_get();
> +
> +		wr.tasks[0] = t1;
> +		wr.cookies[0] = t2->core_task_cookie;
> +
> +	} else {
> +		/* CASE 4. */
> +		sched_core_get_task_cookie(t2->core_task_cookie);
> +		sched_core_get();
> +
> +		sched_core_put_task_cookie(t1->core_task_cookie);
> +		sched_core_put_after_stopper = true;
> +
> +		wr.tasks[0] = t1;
> +		wr.cookies[0] = t2->core_task_cookie;
> +	}
> +
> +	stop_machine(sched_core_task_join_stopper, (void *)&wr, NULL);
> +
> +	if (sched_core_put_after_stopper)
> +		sched_core_put();
> +
> +	ret = 0;
> +out_unlock:
> +	mutex_unlock(&sched_core_tasks_mutex);
> +	return ret;
> +}
> +
> +/* Called from prctl interface: PR_SCHED_CORE_SHARE */
> +int sched_core_share_pid(unsigned long flags, pid_t pid)
> +{
> +	struct task_struct *to;
> +	struct task_struct *from;
> +	struct task_struct *task;
> +	int err;
> +
> +	rcu_read_lock();
> +	task = find_task_by_vpid(pid);
> +	if (!task) {
> +		rcu_read_unlock();
> +		return -ESRCH;
> +	}
> +
> +	get_task_struct(task);
> +
> +	/*
> +	 * Check if this process has the right to modify the specified
> +	 * process. Use the regular "ptrace_may_access()" checks.
> +	 */
> +	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
> +		rcu_read_unlock();
> +		err = -EPERM;
> +		goto out;
> +	}
> +	rcu_read_unlock();
> +
> +	if (flags == PR_SCHED_CORE_CLEAR) {
> +		to = task;
> +		from = NULL;
> +	} else if (flags == PR_SCHED_CORE_SHARE_TO) {
> +		to = task;
> +		from = current;
> +	} else if (flags == PR_SCHED_CORE_SHARE_FROM) {
> +		to = current;
> +		from = task;
> +	} else {
> +		err = -EINVAL;
> +		goto out;
> +	}
> +
> +	err = sched_core_share_tasks(to, from);
> +out:
> +	if (task)
> +		put_task_struct(task);
> +	return err;
> +}
> +
> +/* CGroup core-scheduling interface support. */
> +#ifdef CONFIG_CGROUP_SCHED
> +/*
> + * Helper to get the group cookie in a hierarchy. Any ancestor can have a
> + * cookie.
> + *
> + * Sets *group_cookie_ptr to the hierarchical group cookie.
> + */
> +void cpu_core_get_group_cookie(struct task_group *tg,
> +			       unsigned long *group_cookie_ptr)
> +{
> +	unsigned long group_cookie = 0UL;
> +
> +	if (!tg)
> +		goto out;
> +
> +	for (; tg; tg = tg->parent) {
> +
> +		if (tg->core_tagged) {
> +			group_cookie = (unsigned long)tg;
> +			break;
> +		}
> +	}
> +
> +out:
> +	*group_cookie_ptr = group_cookie;
> +}
> +
> +/* Determine if any group in @tg's children are tagged. */
> +static bool cpu_core_check_descendants(struct task_group *tg, bool check_tag)
> +{
> +	struct task_group *child;
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(child, &tg->children, siblings) {
> +		if ((child->core_tagged && check_tag)) {
> +			rcu_read_unlock();
> +			return true;
> +		}
> +
> +		rcu_read_unlock();
> +		return cpu_core_check_descendants(child, check_tag);
> +	}
> +
> +	rcu_read_unlock();
> +	return false;
> +}
> +
> +u64 cpu_core_tag_read_u64(struct cgroup_subsys_state *css,
> +			  struct cftype *cft)
> +{
> +	struct task_group *tg = css_tg(css);
> +
> +	return !!tg->core_tagged;
> +}
> +
> +#ifdef CONFIG_SCHED_DEBUG
> +u64 cpu_core_group_cookie_read_u64(struct cgroup_subsys_state *css,
> +				   struct cftype *cft)
> +{
> +	unsigned long group_cookie;
> +
> +	cpu_core_get_group_cookie(css_tg(css), &group_cookie);
> +
> +	return group_cookie;
> +}
> +#endif
> +
> +struct write_core_tag {
> +	struct cgroup_subsys_state *css;
> +	unsigned long cookie;
> +	enum sched_core_cookie_type cookie_type;
> +};
> +
> +static int __sched_write_tag(void *data)
> +{
> +	struct write_core_tag *tag = (struct write_core_tag *)data;
> +	struct task_struct *p;
> +	struct cgroup_subsys_state *css;
> +
> +	rcu_read_lock();
> +	css_for_each_descendant_pre(css, tag->css) {
> +		struct css_task_iter it;
> +
> +		css_task_iter_start(css, 0, &it);
> +		/*
> +		 * Note: css_task_iter_next will skip dying tasks.
> +		 * There could still be dying tasks left in the core queue
> +		 * when we set cgroup tag to 0 when the loop is done below.
> +		 */
> +		while ((p = css_task_iter_next(&it)))
> +			sched_core_update_cookie(p, tag->cookie,
> +						 tag->cookie_type);
> +
> +		css_task_iter_end(&it);
> +	}
> +	rcu_read_unlock();
> +
> +	return 0;
> +}
> +
> +int cpu_core_tag_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
> +			   u64 val)
> +{
> +	struct task_group *tg = css_tg(css);
> +	struct write_core_tag wtag;
> +	unsigned long group_cookie;
> +
> +	if (val > 1)
> +		return -ERANGE;
> +
> +	if (!static_branch_likely(&sched_smt_present))
> +		return -EINVAL;
> +
> +	if (!tg->core_tagged && val) {
> +		/* Tag is being set. Check ancestors and descendants. */
> +		cpu_core_get_group_cookie(tg, &group_cookie);
> +		if (group_cookie ||
> +		    cpu_core_check_descendants(tg, true /* tag */))
> +			return -EBUSY;
> +	} else if (tg->core_tagged && !val) {
> +		/* Tag is being reset. Check descendants. */
> +		if (cpu_core_check_descendants(tg, true /* tag */))
> +			return -EBUSY;
> +	} else {
> +		return 0;
> +	}
> +
> +	if (!!val)
> +		sched_core_get();
> +
> +	wtag.css = css;
> +	wtag.cookie = (unsigned long)tg;
> +	wtag.cookie_type = sched_core_group_cookie_type;
> +
> +	tg->core_tagged = val;
> +
> +	stop_machine(__sched_write_tag, (void *)&wtag, NULL);
> +	if (!val)
> +		sched_core_put();
> +
> +	return 0;
> +}
> +#endif
> +
> +/*
> + * Tagging support when fork(2) is called:
> + * If it is a CLONE_THREAD fork, share parent's tag. Otherwise assign a unique per-task tag.
> + */
> +static int sched_update_core_tag_stopper(void *data)
> +{
> +	struct task_struct *p = (struct task_struct *)data;
> +
> +	/* Recalculate core cookie */
> +	sched_core_update_cookie(p, 0, sched_core_no_update);
> +
> +	return 0;
> +}
> +
> +/* Called from sched_fork() */
> +int sched_core_fork(struct task_struct *p, unsigned long clone_flags)
> +{
> +	struct sched_core_cookie *parent_cookie =
> +		(struct sched_core_cookie *)current->core_cookie;
> +
> +	/*
> +	 * core_cookie is ref counted; avoid an uncounted reference.
> +	 * If p should have a cookie, it will be set below.
> +	 */
> +	p->core_cookie = 0UL;
> +
> +	/*
> +	 * If parent is tagged via per-task cookie, tag the child (either with
> +	 * the parent's cookie, or a new one).
> +	 *
> +	 * We can return directly in this case, because sched_core_share_tasks()
> +	 * will set the core_cookie (so there is no need to try to inherit from
> +	 * the parent). The cookie will have the proper sub-fields (ie. group
> +	 * cookie, etc.), because these come from p's task_struct, which is
> +	 * dup'd from the parent.
> +	 */
> +	if (current->core_task_cookie) {
> +		int ret;
> +
> +		/* If it is not CLONE_THREAD fork, assign a unique per-task tag. */
> +		if (!(clone_flags & CLONE_THREAD)) {
> +			ret = sched_core_share_tasks(p, p);
> +		} else {
> +			/* Otherwise share the parent's per-task tag. */
> +			ret = sched_core_share_tasks(p, current);
> +		}
> +
> +		if (ret)
> +			return ret;
> +
> +		/*
> +		 * We expect sched_core_share_tasks() to always update p's
> +		 * core_cookie.
> +		 */
> +		WARN_ON_ONCE(!p->core_cookie);
> +
> +		return 0;
> +	}
> +
> +	/*
> +	 * If parent is tagged, inherit the cookie and ensure that the reference
> +	 * count is updated.
> +	 *
> +	 * Technically, we could instead zero-out the task's group_cookie and
> +	 * allow sched_core_change_group() to handle this post-fork, but
> +	 * inheriting here has a performance advantage, since we don't
> +	 * need to traverse the core_cookies RB tree and can instead grab the
> +	 * parent's cookie directly.
> +	 */
> +	if (parent_cookie) {
> +		bool need_stopper = false;
> +		unsigned long flags;
> +
> +		/*
> +		 * cookies lock prevents task->core_cookie from changing or
> +		 * being freed
> +		 */
> +		raw_spin_lock_irqsave(&sched_core_cookies_lock, flags);
> +
> +		if (likely(refcount_inc_not_zero(&parent_cookie->refcnt))) {
> +			p->core_cookie = (unsigned long)parent_cookie;
> +		} else {
> +			/*
> +			 * Raced with put(). We'll use stop_machine to get
> +			 * a core_cookie
> +			 */
> +			need_stopper = true;
> +		}
> +
> +		raw_spin_unlock_irqrestore(&sched_core_cookies_lock, flags);
> +
> +		if (need_stopper)
> +			stop_machine(sched_update_core_tag_stopper,
> +				     (void *)p, NULL);
> +	}
> +
> +	return 0;
> +}
> +
> +void sched_tsk_free(struct task_struct *tsk)
> +{
> +	struct sched_core_task_cookie *ck;
> +
> +	sched_core_put_cookie((struct sched_core_cookie *)tsk->core_cookie);
> +
> +	if (!tsk->core_task_cookie)
> +		return;
> +
> +	ck = (struct sched_core_task_cookie *)tsk->core_task_cookie;
> +	queue_work(system_wq, &ck->work);
> +}
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 60a922d3f46f..8c452b8010ad 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1024,6 +1024,10 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
>   		__PS("clock-delta", t1-t0);
>   	}
>   
> +#ifdef CONFIG_SCHED_CORE
> +	__PS("core_cookie", p->core_cookie);
> +#endif
> +
>   	sched_show_numa(p, m);
>   }
>   
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index b03f261b95b3..94e07c271528 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -377,6 +377,10 @@ struct cfs_bandwidth {
>   struct task_group {
>   	struct cgroup_subsys_state css;
>   
> +#ifdef CONFIG_SCHED_CORE
> +	int			core_tagged;
> +#endif
> +
>   #ifdef CONFIG_FAIR_GROUP_SCHED
>   	/* schedulable entities of this group on each CPU */
>   	struct sched_entity	**se;
> @@ -425,6 +429,11 @@ struct task_group {
>   
>   };
>   
> +static inline struct task_group *css_tg(struct cgroup_subsys_state *css)
> +{
> +	return css ? container_of(css, struct task_group, css) : NULL;
> +}
> +
>   #ifdef CONFIG_FAIR_GROUP_SCHED
>   #define ROOT_TASK_GROUP_LOAD	NICE_0_LOAD
>   
> @@ -1127,6 +1136,12 @@ static inline bool is_migration_disabled(struct task_struct *p)
>   DECLARE_STATIC_KEY_FALSE(__sched_core_enabled);
>   static inline struct cpumask *sched_group_span(struct sched_group *sg);
>   
> +enum sched_core_cookie_type {
> +	sched_core_no_update = 0,
> +	sched_core_task_cookie_type,
> +	sched_core_group_cookie_type,
> +};
> +
>   static inline bool sched_core_enabled(struct rq *rq)
>   {
>   	return static_branch_unlikely(&__sched_core_enabled) && rq->core_enabled;
> @@ -1197,12 +1212,53 @@ static inline bool sched_group_cookie_match(struct rq *rq,
>   	return false;
>   }
>   
> -extern void queue_core_balance(struct rq *rq);
> +void sched_core_change_group(struct task_struct *p, struct task_group *new_tg);
> +int sched_core_fork(struct task_struct *p, unsigned long clone_flags);
> +
> +static inline bool sched_core_enqueued(struct task_struct *task)
> +{
> +	return !RB_EMPTY_NODE(&task->core_node);
> +}
> +
> +void queue_core_balance(struct rq *rq);
> +
> +void sched_core_enqueue(struct rq *rq, struct task_struct *p);
> +void sched_core_dequeue(struct rq *rq, struct task_struct *p);
> +void sched_core_get(void);
> +void sched_core_put(void);
> +
> +int sched_core_share_pid(unsigned long flags, pid_t pid);
> +int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2);
> +
> +#ifdef CONFIG_CGROUP_SCHED
> +u64 cpu_core_tag_read_u64(struct cgroup_subsys_state *css,
> +			  struct cftype *cft);
> +
> +#ifdef CONFIG_SCHED_DEBUG
> +u64 cpu_core_group_cookie_read_u64(struct cgroup_subsys_state *css,
> +				   struct cftype *cft);
> +#endif
> +
> +int cpu_core_tag_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
> +			   u64 val);
> +#endif
> +
> +#ifndef TIF_UNSAFE_RET
> +#define TIF_UNSAFE_RET (0)
> +#endif
>   
>   bool cfs_prio_less(struct task_struct *a, struct task_struct *b, bool fi);
>   
>   #else /* !CONFIG_SCHED_CORE */
>   
> +static inline bool sched_core_enqueued(struct task_struct *task) { return false; }
> +static inline void sched_core_enqueue(struct rq *rq, struct task_struct *p) { }
> +static inline void sched_core_dequeue(struct rq *rq, struct task_struct *p) { }
> +static inline int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)
> +{
> +	return -ENOTSUPP;
> +}
> +
>   static inline bool sched_core_enabled(struct rq *rq)
>   {
>   	return false;
> @@ -2899,7 +2955,4 @@ void swake_up_all_locked(struct swait_queue_head *q);
>   void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait);
>   
>   #ifdef CONFIG_SCHED_CORE
> -#ifndef TIF_UNSAFE_RET
> -#define TIF_UNSAFE_RET (0)
> -#endif
>   #endif
> diff --git a/kernel/sys.c b/kernel/sys.c
> index a730c03ee607..da52a0da24ef 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2530,6 +2530,13 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>   
>   		error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER;
>   		break;
> +#ifdef CONFIG_SCHED_CORE
> +	case PR_SCHED_CORE_SHARE:
> +		if (arg4 || arg5)
> +			return -EINVAL;
> +		error = sched_core_share_pid(arg2, arg3);
> +		break;
> +#endif
>   	default:
>   		error = -EINVAL;
>   		break;
> diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h
> index 7f0827705c9a..8fbf9d164ec4 100644
> --- a/tools/include/uapi/linux/prctl.h
> +++ b/tools/include/uapi/linux/prctl.h
> @@ -247,4 +247,10 @@ struct prctl_mm_map {
>   #define PR_SET_IO_FLUSHER		57
>   #define PR_GET_IO_FLUSHER		58
>   
> +/* Request the scheduler to share a core */
> +#define PR_SCHED_CORE_SHARE		59
> +#define PR_SCHED_CORE_CLEAR		0  /* clear core_sched cookie of pid */
> +#define PR_SCHED_CORE_SHARE_FROM	1  /* get core_sched cookie from pid */
> +#define PR_SCHED_CORE_SHARE_TO		2  /* push core_sched cookie to pid */

My original patch had this:

# define PR_SCHED_CORE_CLEAR            0  /* clear core_sched cookie of pid */
# define PR_SCHED_CORE_SHARE_FROM       1  /* get core_sched cookie from pid */
# define PR_SCHED_CORE_SHARE_TO         2  /* push core_sched cookie to pid */

The space between the '#' and 'define' is the trick apparently used to prevent a script run during 'perf' build that 
pulls options out of this file to generate a 'beauty' array. Without the space, you can't build perf.

-chrish

> +
>   #endif /* _LINUX_PRCTL_H */