Re: [PATCH v2] sched: cpuset: Don't rebuild sched domains on suspend-resume

[Waiman Long]

On 1/25/23 11:35, Qais Yousef wrote:
> On 01/20/23 17:16, Waiman Long wrote:
> > On 1/20/23 14:48, Qais Yousef wrote:
> > > Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
> > > enabled rebuilding sched domain on cpuset and hotplug operations to
> > > correct deadline accounting.
> > >
> > > Rebuilding sched domain is a slow operation and we see 10+ ms delay on
> > > suspend-resume because of that.
> > >
> > > Since nothing is expected to change on suspend-resume operation; skip
> > > rebuilding the sched domains to regain the time lost.
> > >
> > > Debugged-by: Rick Yiu <rickyiu@xxxxxxxxxx>
> > > Signed-off-by: Qais Yousef (Google) <qyousef@xxxxxxxxxxx>
> > > ---
> > >
> > >       Changes in v2:
> > >       	* Remove redundant check in update_tasks_root_domain() (Thanks Waiman)
> > >       v1 link:
> > >       	https://lore.kernel.org/lkml/20221216233501.gh6m75e7s66dmjgo@airbuntu/
> > >
> > >    kernel/cgroup/cpuset.c  | 3 +++
> > >    kernel/sched/deadline.c | 3 +++
> > >    2 files changed, 6 insertions(+)
> > >
> > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > > index a29c0b13706b..9a45f083459c 100644
> > > --- a/kernel/cgroup/cpuset.c
> > > +++ b/kernel/cgroup/cpuset.c
> > > @@ -1088,6 +1088,9 @@ static void rebuild_root_domains(void)
> > >    	lockdep_assert_cpus_held();
> > >    	lockdep_assert_held(&sched_domains_mutex);
> > > +	if (cpuhp_tasks_frozen)
> > > +		return;
> > > +
> > >    	rcu_read_lock();
> > >    	/*
> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > index 0d97d54276cc..42c1143a3956 100644
> > > --- a/kernel/sched/deadline.c
> > > +++ b/kernel/sched/deadline.c
> > > @@ -2575,6 +2575,9 @@ void dl_clear_root_domain(struct root_domain *rd)
> > >    {
> > >    	unsigned long flags;
> > > +	if (cpuhp_tasks_frozen)
> > > +		return;
> > > +
> > >    	raw_spin_lock_irqsave(&rd->dl_bw.lock, flags);
> > >    	rd->dl_bw.total_bw = 0;
> > >    	raw_spin_unlock_irqrestore(&rd->dl_bw.lock, flags);
> > cpuhp_tasks_frozen is set when thaw_secondary_cpus() or
> > freeze_secondary_cpus() is called. I don't know the exact suspend/resume
> > calling sequences, will cpuhp_tasks_frozen be cleared at the end of resume
> > sequence? Maybe we should make sure that rebuild_root_domain() is called at
> > least once at the end of resume operation.
> Very good questions. It made me look at the logic again and I realize now that
> the way force_build behaves is causing this issue.
>
> I *think* we should just make the call rebuild_root_domains() only if
> cpus_updated in cpuset_hotplug_workfn().
>
> cpuset_cpu_active() seems to be the source of force_rebuild in my case; which
> seems to be called only after the last cpu is back online (what you suggest).
> In this case we can end up with cpus_updated = false, but force_rebuild = true.
>
> Now you added a couple of new users to force_rebuild in 4b842da276a8a; I'm
> trying to figure out what the conditions would be there. It seems we can have
> corner cases for cpus_update might not trigger correctly?
>
> Could the below be a good cure?
>
> AFAICT we must rebuild the root domains if something has changed in cpuset.
> Which should be captured by either having:
>
> 	* cpus_updated = true
> 	* force_rebuild && !cpuhp_tasks_frozen
>
> /me goes to test the patch
>
> --->8---
>
> 	diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> 	index a29c0b13706b..363e4459559f 100644
> 	--- a/kernel/cgroup/cpuset.c
> 	+++ b/kernel/cgroup/cpuset.c
> 	@@ -1079,6 +1079,8 @@ static void update_tasks_root_domain(struct cpuset *cs)
> 		css_task_iter_end(&it);
> 	 }
>
> 	+static bool need_rebuild_rd = true;
> 	+
> 	 static void rebuild_root_domains(void)
> 	 {
> 		struct cpuset *cs = NULL;
> 	@@ -1088,6 +1090,9 @@ static void rebuild_root_domains(void)
> 		lockdep_assert_cpus_held();
> 		lockdep_assert_held(&sched_domains_mutex);
>
> 	+       if (!need_rebuild_rd)
> 	+               return;
> 	+
> 		rcu_read_lock();
>
> 		/*
> 	@@ -3627,7 +3632,9 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
> 		/* rebuild sched domains if cpus_allowed has changed */
> 		if (cpus_updated || force_rebuild) {
> 			force_rebuild = false;
> 	+               need_rebuild_rd = cpus_updated || (force_rebuild && !cpuhp_tasks_frozen);
> 			rebuild_sched_domains();
> 	+               need_rebuild_rd = true;

You do the force_check check after it is set to false in the previous 
statement which is definitely not correct. So it will be false whenever 
cpus_updated is false.

If you just want to skip rebuild_sched_domains() call for hotplug, why 
don't just skip the call here if the condition is right? Like

        /* rebuild sched domains if cpus_allowed has changed */
        if (cpus_updated || (force_rebuild && !cpuhp_tasks_frozen)) {
                force_rebuild = false;
                rebuild_sched_domains();
        }

Still, we will need to confirm that cpuhp_tasks_frozen will be cleared 
outside of the suspend/resume cycle.

Cheers,
Longman