Re: [RFC PATCH 1/2] sched: Rate limit migrations to 1 per 2ms per task

[Tim Chen]

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;
> +}
> +

Tim