Re: [RFC PATCH v3 09/16] sched: Introduce sched_class::pick_task()

From: mark gross
Date: Mon Aug 26 2019 - 13:14:54 EST

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On Wed, May 29, 2019 at 08:36:45PM +0000, Vineeth Remanan Pillai wrote:
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>
> Because sched_class::pick_next_task() also implies
> sched_class::set_next_task() (and possibly put_prev_task() and
> newidle_balance) it is not state invariant. This makes it unsuitable
> for remote task selection.
It would be cool if the commit comment would explain what the change is going
to do about pick_next_task being unsuitable for remote task selection.

>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Vineeth Remanan Pillai <vpillai@xxxxxxxxxxxxxxxx>
> Signed-off-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxxxxxx>
> ---
>
> Chnages in v3
> -------------
> - Minor refactor to remove redundant NULL checks
>
> Changes in v2
> -------------
> - Fixes a NULL pointer dereference crash
> - Subhra Mazumdar
> - Tim Chen
>
> ---
> kernel/sched/deadline.c | 21 ++++++++++++++++-----
> kernel/sched/fair.c | 36 +++++++++++++++++++++++++++++++++---
> kernel/sched/idle.c | 10 +++++++++-
> kernel/sched/rt.c | 21 ++++++++++++++++-----
> kernel/sched/sched.h | 2 ++
> kernel/sched/stop_task.c | 21 ++++++++++++++++-----
> 6 files changed, 92 insertions(+), 19 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index d3904168857a..64fc444f44f9 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1722,15 +1722,12 @@ static struct sched_dl_entity *pick_next_dl_entity(struct rq *rq,
> return rb_entry(left, struct sched_dl_entity, rb_node);
> }
>
> -static struct task_struct *
> -pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> +static struct task_struct *pick_task_dl(struct rq *rq)
> {
> struct sched_dl_entity *dl_se;
> struct task_struct *p;
> struct dl_rq *dl_rq;
>
> - WARN_ON_ONCE(prev || rf);
> -
> dl_rq = &rq->dl;
>
> if (unlikely(!dl_rq->dl_nr_running))
> @@ -1741,7 +1738,19 @@ pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>
> p = dl_task_of(dl_se);
>
> - set_next_task_dl(rq, p);
> + return p;
> +}
> +
> +static struct task_struct *
> +pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> +{
> + struct task_struct *p;
> +
> + WARN_ON_ONCE(prev || rf);
What is an admin to do with this warnding if it shows up in there logs?
maybe include some text here to help folks that might hit this warn_on.

> +
> + p = pick_task_dl(rq);
> + if (p)
> + set_next_task_dl(rq, p);
>
> return p;
> }
> @@ -2388,6 +2397,8 @@ const struct sched_class dl_sched_class = {
> .set_next_task = set_next_task_dl,
>
> #ifdef CONFIG_SMP
> + .pick_task = pick_task_dl,
> +
> .select_task_rq = select_task_rq_dl,
> .migrate_task_rq = migrate_task_rq_dl,
> .set_cpus_allowed = set_cpus_allowed_dl,
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e65f2dfda77a..02e5dfb85e7d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4136,7 +4136,7 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> * Avoid running the skip buddy, if running something else can
> * be done without getting too unfair.
> */
> - if (cfs_rq->skip == se) {
> + if (cfs_rq->skip && cfs_rq->skip == se) {
> struct sched_entity *second;
>
> if (se == curr) {
> @@ -4154,13 +4154,13 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> /*
> * Prefer last buddy, try to return the CPU to a preempted task.
> */
> - if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
> + if (left && cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
> se = cfs_rq->last;
>
> /*
> * Someone really wants this to run. If it's not unfair, run it.
> */
> - if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
> + if (left && cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
> se = cfs_rq->next;
>
> clear_buddies(cfs_rq, se);
> @@ -6966,6 +6966,34 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
> set_last_buddy(se);
> }
>
> +static struct task_struct *
> +pick_task_fair(struct rq *rq)
> +{
> + struct cfs_rq *cfs_rq = &rq->cfs;
> + struct sched_entity *se;
> +
> + if (!cfs_rq->nr_running)
> + return NULL;
> +
> + do {
> + struct sched_entity *curr = cfs_rq->curr;
> +
> + se = pick_next_entity(cfs_rq, NULL);
> +
> + if (curr) {
> + if (se && curr->on_rq)
> + update_curr(cfs_rq);
> +
> + if (!se || entity_before(curr, se))
> + se = curr;
> + }
> +
> + cfs_rq = group_cfs_rq(se);
> + } while (cfs_rq);
> +
> + return task_of(se);
> +}
> +
> static struct task_struct *
> pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> {
> @@ -10677,6 +10705,8 @@ const struct sched_class fair_sched_class = {
> .set_next_task = set_next_task_fair,
>
> #ifdef CONFIG_SMP
> + .pick_task = pick_task_fair,
> +
> .select_task_rq = select_task_rq_fair,
> .migrate_task_rq = migrate_task_rq_fair,
>
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 7ece8e820b5d..e7f38da60373 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -373,6 +373,12 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
> resched_curr(rq);
> }
>
> +static struct task_struct *
> +pick_task_idle(struct rq *rq)
> +{
> + return rq->idle;
> +}
> +
> static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> {
> }
> @@ -386,11 +392,12 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next)
> static struct task_struct *
> pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> {
> - struct task_struct *next = rq->idle;
> + struct task_struct *next;
>
> if (prev)
> put_prev_task(rq, prev);
>
> + next = pick_task_idle(rq);
> set_next_task_idle(rq, next);
>
> return next;
> @@ -458,6 +465,7 @@ const struct sched_class idle_sched_class = {
> .set_next_task = set_next_task_idle,
>
> #ifdef CONFIG_SMP
> + .pick_task = pick_task_idle,
> .select_task_rq = select_task_rq_idle,
> .set_cpus_allowed = set_cpus_allowed_common,
> #endif
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 79f2e60516ef..81557224548c 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1548,20 +1548,29 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
> return rt_task_of(rt_se);
> }
>
> -static struct task_struct *
> -pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> +static struct task_struct *pick_task_rt(struct rq *rq)
> {
> struct task_struct *p;
> struct rt_rq *rt_rq = &rq->rt;
>
> - WARN_ON_ONCE(prev || rf);
> -
> if (!rt_rq->rt_queued)
> return NULL;
>
> p = _pick_next_task_rt(rq);
>
> - set_next_task_rt(rq, p);
> + return p;
> +}
> +
> +static struct task_struct *
> +pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> +{
> + struct task_struct *p;
> +
> + WARN_ON_ONCE(prev || rf);
what does it mean if this warn on goes off to an admin?

> +
> + p = pick_task_rt(rq);
> + if (p)
> + set_next_task_rt(rq, p);
>
> return p;
> }
> @@ -2364,6 +2373,8 @@ const struct sched_class rt_sched_class = {
> .set_next_task = set_next_task_rt,
>
> #ifdef CONFIG_SMP
> + .pick_task = pick_task_rt,
> +
> .select_task_rq = select_task_rq_rt,
>
> .set_cpus_allowed = set_cpus_allowed_common,
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 460dd04e76af..a024dd80eeb3 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1682,6 +1682,8 @@ struct sched_class {
> void (*set_next_task)(struct rq *rq, struct task_struct *p);
>
> #ifdef CONFIG_SMP
> + struct task_struct * (*pick_task)(struct rq *rq);
> +
> int (*select_task_rq)(struct task_struct *p, int task_cpu, int sd_flag, int flags);
> void (*migrate_task_rq)(struct task_struct *p, int new_cpu);
>
> diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
> index 7e1cee4e65b2..fb6c436cba6c 100644
> --- a/kernel/sched/stop_task.c
> +++ b/kernel/sched/stop_task.c
> @@ -29,20 +29,30 @@ static void set_next_task_stop(struct rq *rq, struct task_struct *stop)
> }
>
> static struct task_struct *
> -pick_next_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> +pick_task_stop(struct rq *rq)
> {
> struct task_struct *stop = rq->stop;
>
> - WARN_ON_ONCE(prev || rf);
> -
> if (!stop || !task_on_rq_queued(stop))
> return NULL;
>
> - set_next_task_stop(rq, stop);
> -
> return stop;
> }
>
> +static struct task_struct *
> +pick_next_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> +{
> + struct task_struct *p;
> +
> + WARN_ON_ONCE(prev || rf);
> +
> + p = pick_task_stop(rq);
> + if (p)
> + set_next_task_stop(rq, p);
> +
> + return p;
> +}
> +
> static void
> enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
> {
> @@ -129,6 +139,7 @@ const struct sched_class stop_sched_class = {
> .set_next_task = set_next_task_stop,
>
> #ifdef CONFIG_SMP
> + .pick_task = pick_task_stop,
> .select_task_rq = select_task_rq_stop,
> .set_cpus_allowed = set_cpus_allowed_common,
> #endif
> --
> 2.17.1
>

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