Re: [PATCH v3 1/2] sched/numa: Drive NUMA task tick from execution context

[Hui Su]

On Tue, Sep 08, 2026 at 12:44:07PM +0200, Peter Zijlstra wrote:
> I also wondered if we should have task_tick() do curr_class->task_tick()
> last, rather than first. But I couldn't immediately find a compelling
> argument either way around.
>
> Updated...
>
> @@ -910,6 +910,16 @@ static void __used hrtick_clear(struct rq *rq)
>  		hrtimer_cancel(&rq->hrtick_timer);
>  }
>
> +static inline void task_tick(struct rq *rq, int queued)
> +{
> +	const struct sched_class *curr_class = rq->curr->sched_class,
> +				*donor_class = rq->donor->sched_class;
> +
> +	curr_class->task_tick(rq, queued);
> +	if (sched_proxy_exec() && donor_class != curr_class)
> +		donor_class->task_tick(rq, queued);
> +}

The per-class split looks good to me. I noticed a couple of details while
going through the updated version.

First, I think there is a reason to call curr_class->task_tick() last.

For example, with an RT/DL donor and a FAIR execution context, the donor
class tick calls update_curr_*(), which goes through
update_curr_common()/update_se() and accounts the elapsed task runtime
to rq->curr->se.sum_exec_runtime. If the FAIR callback runs first,
task_tick_numa() observes sum_exec_runtime before the current tick's runtime
has been accounted.

Calling the donor class first preserves the accounting-before-consumer
ordering:

	donor_class->task_tick(rq, queued);
	if (sched_proxy_exec() && curr_class != donor_class)
		curr_class->task_tick(rq, queued);

This also matches the ordering in v3, where the donor tick ran before
sched_tick_exec_ctx().

Second, task_tick() is added after hrtick_clear(), which puts it inside
CONFIG_SCHED_HRTICK. Since sched_tick() calls task_tick() unconditionally,
CONFIG_SCHED_HRTICK=n would leave task_tick() undefined. The helper needs
to be outside the CONFIG_SCHED_HRTICK block.

With those changes, the split and the queued handling look good to me so far.
I'll test the cross-class and hrtick cases, rework the series accordingly,
and send a v4.

Thanks,
Hui