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SubjectRe: [RFC PATCH 1/2] sched: Rate limit migrations to 1 per 2ms per task
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On 9/6/23 16:51, Tim Chen wrote:
> On Wed, 2023-09-06 at 11:47 +0200, Peter Zijlstra wrote:
>> On Tue, Sep 05, 2023 at 03:44:57PM -0700, Tim Chen wrote:
>>
>>> Reading up on sched_clock() documentation and seems like it should
>>> indeed be monotonic.
>>
>> It tries very hard to be monotonic but cannot guarantee. The moment TSC
>> is found unstable it's too late to fix up everything.
>>
>
> Yes, if TSC becomes unstable and could cause sched_clock to reset and go way backward.
> Perhaps we can add the following check in Mathieu's original
> patch to fix things up:
>
> +static bool should_migrate_task(struct task_struct *p, int prev_cpu)
>> +{
> /* sched_clock reset causing next migration time to be too far ahead */
> if (p->se.next_migration_time > sched_clock_cpu(prev_cpu) + SCHED_MIGRATION_RATELIMIT_WINDOW)
> p->se.next_migration_time = sched_clock_cpu(prev_cpu) + SCHED_MIGRATION_RATELIMIT_WINDOW;
>
>> + /* Rate limit task migration. */
>> + if (sched_clock_cpu(prev_cpu) < p->se.next_migration_time)
>> + return false;
>> + return true;
>> +}
>> +
>

Along those lines I think something like this should work:

static bool should_migrate_task(struct task_struct *p, int prev_cpu)
{
u64 now = sched_clock_cpu(prev_cpu);

/* sched_clock reset causing next migration time to be too far ahead. */
if (now + SCHED_MIGRATION_RATELIMIT_WINDOW < p->se.next_migration_time)
return true;
/* Rate limit task migration. */
if (now >= p->se.next_migration_time)
return true;
return false;
}

It will let migrate_task_rq_fair() update se->next_migration_time.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

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