Re: [RFC PATCH 2/3] sched/cpuset: Keep track of SCHED_DEADLINE tasks in cpusets

[Juri Lelli]

On 15/03/23 11:46, Waiman Long wrote:
> 
> On 3/15/23 08:18, Juri Lelli wrote:
> > Qais reported that iterating over all tasks when rebuilding root domains
> > for finding out which ones are DEADLINE and need their bandwidth
> > correctly restored on such root domains can be a costly operation (10+
> > ms delays on suspend-resume).
> > 
> > To fix the problem keep track of the number of DEADLINE tasks belonging
> > to each cpuset and then use this information (followup patch) to only
> > perform the above iteration if DEADLINE tasks are actually present in
> > the cpuset for which a corresponding root domain is being rebuilt.
> > 
> > Reported-by: Qais Yousef <qyousef@xxxxxxxxxxx>
> > Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
> > ---
> >   include/linux/cpuset.h |  4 ++++
> >   kernel/cgroup/cgroup.c |  4 ++++
> >   kernel/cgroup/cpuset.c | 25 +++++++++++++++++++++++++
> >   kernel/sched/core.c    | 10 ++++++++++
> >   4 files changed, 43 insertions(+)
> > 
> > diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> > index 355f796c5f07..0348dba5680e 100644
> > --- a/include/linux/cpuset.h
> > +++ b/include/linux/cpuset.h
> > @@ -71,6 +71,8 @@ extern void cpuset_init_smp(void);
> >   extern void cpuset_force_rebuild(void);
> >   extern void cpuset_update_active_cpus(void);
> >   extern void cpuset_wait_for_hotplug(void);
> > +extern void inc_dl_tasks_cs(struct task_struct *task);
> > +extern void dec_dl_tasks_cs(struct task_struct *task);
> >   extern void cpuset_lock(void);
> >   extern void cpuset_unlock(void);
> >   extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
> > @@ -196,6 +198,8 @@ static inline void cpuset_update_active_cpus(void)
> >   static inline void cpuset_wait_for_hotplug(void) { }
> > +static inline void inc_dl_tasks_cs(struct task_struct *task) { }
> > +static inline void dec_dl_tasks_cs(struct task_struct *task) { }
> >   static inline void cpuset_lock(void) { }
> >   static inline void cpuset_unlock(void) { }
> > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > index c099cf3fa02d..357925e1e4af 100644
> > --- a/kernel/cgroup/cgroup.c
> > +++ b/kernel/cgroup/cgroup.c
> > @@ -57,6 +57,7 @@
> >   #include <linux/file.h>
> >   #include <linux/fs_parser.h>
> >   #include <linux/sched/cputime.h>
> > +#include <linux/sched/deadline.h>
> >   #include <linux/psi.h>
> >   #include <net/sock.h>
> > @@ -6673,6 +6674,9 @@ void cgroup_exit(struct task_struct *tsk)
> >   	list_add_tail(&tsk->cg_list, &cset->dying_tasks);
> >   	cset->nr_tasks--;
> > +	if (dl_task(tsk))
> > +		dec_dl_tasks_cs(tsk);
> > +
> >   	WARN_ON_ONCE(cgroup_task_frozen(tsk));
> >   	if (unlikely(!(tsk->flags & PF_KTHREAD) &&
> >   		     test_bit(CGRP_FREEZE, &task_dfl_cgroup(tsk)->flags)))
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index 8d82d66d432b..57bc60112618 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -193,6 +193,12 @@ struct cpuset {
> >   	int use_parent_ecpus;
> >   	int child_ecpus_count;
> > +	/*
> > +	 * number of SCHED_DEADLINE tasks attached to this cpuset, so that we
> > +	 * know when to rebuild associated root domain bandwidth information.
> > +	 */
> > +	int nr_deadline_tasks;
> > +
> >   	/* Invalid partition error code, not lock protected */
> >   	enum prs_errcode prs_err;
> > @@ -245,6 +251,20 @@ static inline struct cpuset *parent_cs(struct cpuset *cs)
> >   	return css_cs(cs->css.parent);
> >   }
> > +void inc_dl_tasks_cs(struct task_struct *p)
> > +{
> > +	struct cpuset *cs = task_cs(p);
> > +
> > +	cs->nr_deadline_tasks++;
> > +}
> > +
> > +void dec_dl_tasks_cs(struct task_struct *p)
> > +{
> > +	struct cpuset *cs = task_cs(p);
> > +
> > +	cs->nr_deadline_tasks--;
> > +}
> > +
> >   /* bits in struct cpuset flags field */
> >   typedef enum {
> >   	CS_ONLINE,
> > @@ -2472,6 +2492,11 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
> >   		ret = security_task_setscheduler(task);
> >   		if (ret)
> >   			goto out_unlock;
> > +
> > +		if (dl_task(task)) {
> > +			cs->nr_deadline_tasks++;
> > +			cpuset_attach_old_cs->nr_deadline_tasks--;
> > +		}
> >   	}
> 
> Any one of the tasks in the cpuset can cause the test to fail and abort the
> attachment. I would suggest that you keep a deadline task transfer count in
> the loop and then update cs and cpouset_attach_old_cs only after all the
> tasks have been iterated successfully.

Right, Dietmar I think commented pointing out something along these
lines. Think though we already have this problem with current
task_can_attach -> dl_cpu_busy which reserves bandwidth for each tasks
in the destination cs. Will need to look into that. Do you know which
sort of operation would move multiple tasks at once?