linux-kernel - Re: [PATCH] kernel/sched: Update schedstats when migrating threads

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Date:   Wed, 23 Feb 2022 09:33:59 -0600
From:   Carlos Bilbao <carlos.bilbao@....com>
To:     Phil Auld <pauld@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, mingo@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/sched: Update schedstats when migrating threads

On 2/23/2022 9:28 AM, Phil Auld wrote:
> On Wed, Feb 23, 2022 at 09:14:45AM -0600 Carlos Bilbao wrote:
>> On 2/23/2022 4:19 AM, Peter Zijlstra wrote:
>>> On Wed, Jan 26, 2022 at 09:22:23AM -0600, Carlos Bilbao wrote:
>>>> The kernel manages per-task scheduler statistics or schedstats. Such
>>>> counters should be reinitialized when the thread is migrated to a
>>>> different core rq, except for the values recording number of migrations.
>>>
>>> I'm confused, why should we reset schedstats on migrate? I'm thinking
>>> this breaks per-task, since tasks tend to bounce around quite a lot.
>>>
>>
>> Thanks for your comments, Peter. 
>>
>> Looking at the documentation of schedstats I see that most values are 
>> actually linked to the particular CPU: time spent on the cpu, timeslices 
>> run on this cpu, number of times _something_ was called when the cpu was 
>> idle, and so forth. Those values lose their meaning after migration and we 
>> should reinitialize their counters. However, reviewing sched_statistics I 
>> identify two fields that we should definitely keep increasing even after 
>> migration (nr_migrations_cold, nr_forced_migrations).
>>
> 
> The documentation is a little off. I think it should say "any cpu" instead
> of "this cpu".  If you reset these per task counters (time on cpu, number
> of timeslices etc) on every migration then they become meaningless (and
> useless).
> 
> 
> Cheers,
> Phil
> 

Well that clarifies it! Then, let me ask the opposite question... What
fields of schedstats should we clear when migrating? If there isn't any,
I will just increase the number of migrations.

>> So this patch will have to be upgraded if there's some other value(s) in
>> schedstats that we do not want to reinitialize either.
>>
>>>> Signed-off-by: Carlos Bilbao <carlos.bilbao@....com>
>>>> ---
>>>>  kernel/sched/core.c | 10 +++++++++-
>>>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>>> index fe53e510e711..d64c2a290176 100644
>>>> --- a/kernel/sched/core.c
>>>> +++ b/kernel/sched/core.c
>>>> @@ -8757,6 +8757,7 @@ bool sched_smp_initialized __read_mostly;
>>>>  int migrate_task_to(struct task_struct *p, int target_cpu)
>>>>  {
>>>>  	struct migration_arg arg = { p, target_cpu };
>>>> +	uint64_t forced_migrations, migrations_cold;
>>>>  	int curr_cpu = task_cpu(p);
>>>>  
>>>>  	if (curr_cpu == target_cpu)
>>>> @@ -8765,7 +8766,14 @@ int migrate_task_to(struct task_struct *p, int target_cpu)
>>>>  	if (!cpumask_test_cpu(target_cpu, p->cpus_ptr))
>>>>  		return -EINVAL;
>>>>  
>>>> -	/* TODO: This is not properly updating schedstats */
>>>> +	if (schedstat_enabled()) {
>>>> +		forced_migrations = schedstat_val(p->stats.nr_forced_migrations);
>>>> +		migrations_cold = schedstat_val(p->stats.nr_migrations_cold);
>>>> +		memset(&p->stats, 0, sizeof(p->stats));
>>>> +		schedstat_set(p->stats.nr_forced_migrations, forced_migrations);
>>>> +		schedstat_set(p->stats.nr_migrations_cold, migrations_cold);
>>>> +		schedstat_inc(p->stats.nr_migrations_cold);
>>>> +	}
>>>>  
>>>>  	trace_sched_move_numa(p, curr_cpu, target_cpu);
>>>>  	return stop_one_cpu(curr_cpu, migration_cpu_stop, &arg);
>>>> -- 
>>>> 2.27.0
>>>>
>>
>> Thanks,
>> Carlos
>>
> 

Thanks,
Carlos