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

[Waiman Long]

On 1/29/23 21:49, Waiman Long wrote:
> 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.

BTW, you also need to expand the comment to explain why we need to check 
for cpuhp_tasks_frozen.

Cheers,
Longman