Re: [RFC][PATCH v14 3/7] sched: Fix runtime accounting w/ split exec & sched contexts

Peter Zijlstra
2024-12-13T23:37:50Z

On Mon, Nov 25, 2024 at 11:51:57AM -0800, John Stultz wrote:

> -static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
> +static s64 update_curr_se(struct rq *rq, struct sched_entity *se)
> {
> u64 now = rq_clock_task(rq);
> s64 delta_exec;
>
> - delta_exec = now - curr->exec_start;
> + delta_exec = now - se->exec_start;
> if (unlikely(delta_exec <= 0))
> return delta_exec;
>
> - curr->exec_start = now;
> - curr->sum_exec_runtime += delta_exec;
> + se->exec_start = now;
> + if (entity_is_task(se)) {
> + struct task_struct *running = rq->curr;
> + /*
> + * If se is a task, we account the time against the running
> + * task, as w/ proxy-exec they may not be the same.
> + */
> + running->se.exec_start = now;
> + running->se.sum_exec_runtime += delta_exec;
> + } else {
> + /* If not task, account the time against se */
> + se->sum_exec_runtime += delta_exec;
> + }
>
> if (schedstat_enabled()) {
> struct sched_statistics *stats;
>
> - stats = __schedstats_from_se(curr);
> + stats = __schedstats_from_se(se);
> __schedstat_set(stats->exec_max,
> max(delta_exec, stats->exec_max));
> }

Would it not be *much* clearer if we do it like:

static s64 update_curr_se(struct rq *rq, struct sched_entity *donor,
struct sched_entity *curr)
{
...
donor->exec_start = now;
curr->exec_start = now;
curr->sum_exec_runtime += delta_exec;
...
}

and update the callsites like so:

update_curr_common()
update_curr_se(rq, &donor->se, &rq->curr.se)

update_curr()
update_curr_se(rq, &curr->se, &curr->se);

except, now I'm confused about the update_curr() case. That seems to
always update the execution context, rather than the donor ?