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

[Waiman Long <longman@redhat.com>]

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@google.com>
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
>
>      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.

Cheers,
Longman