Message-ID: <45e0f8ea-d229-1ae7-5c12-7f0a64c6767a@redhat.com>
Date:   Sun, 29 Jan 2023 21:49:49 -0500
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Subject: Re: [PATCH v2] sched: cpuset: Don't rebuild sched domains on
 suspend-resume
Content-Language: en-US
To:     Qais Yousef <qyousef@layalina.io>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>, tj@kernel.org,
        linux-kernel@vger.kernel.org, luca.abeni@santannapisa.it,
        claudio@evidence.eu.com, tommaso.cucinotta@santannapisa.it,
        bristot@redhat.com, mathieu.poirier@linaro.org,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        cgroups@vger.kernel.org,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Wei Wang <wvw@google.com>, Rick Yiu <rickyiu@google.com>,
        Quentin Perret <qperret@google.com>
References: <20230120194822.962958-1-qyousef@layalina.io>
 <c4c2dec6-a72b-d675-fb42-be40e384ea2c@redhat.com>
 <20230125163546.pspvigh4groiwjy7@airbuntu>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20230125163546.pspvigh4groiwjy7@airbuntu>
Content-Type: text/plain; charset=UTF-8; format=flowed
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Precedence: bulk

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@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.
> 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