Re: [PATCH 6/7] psi: pressure stall information for CPU, memory, and IO

Peter Zijlstra
2018-05-09T10:46:18Z

On Mon, May 07, 2018 at 05:01:34PM -0400, Johannes Weiner wrote:

> @@ -2038,6 +2038,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
> if (task_cpu(p) != cpu) {
> wake_flags |= WF_MIGRATED;
> + psi_ttwu_dequeue(p);
> set_task_cpu(p, cpu);
> }
>

> +static inline void psi_ttwu_dequeue(struct task_struct *p)
> +{
> + /*
> + * Is the task being migrated during a wakeup? Make sure to
> + * deregister its sleep-persistent psi states from the old
> + * queue, and let psi_enqueue() know it has to requeue.
> + */
> + if (unlikely(p->in_iowait || (p->flags & PF_MEMSTALL))) {
> + struct rq_flags rf;
> + struct rq *rq;
> + int clear = 0;
> +
> + if (p->in_iowait)
> + clear |= TSK_IOWAIT;
> + if (p->flags & PF_MEMSTALL)
> + clear |= TSK_MEMSTALL;
> +
> + rq = __task_rq_lock(p, &rf);
> + update_rq_clock(rq);
> + psi_task_change(p, rq_clock(rq), clear, 0);
> + p->sched_psi_wake_requeue = 1;
> + __task_rq_unlock(rq, &rf);
> + }
> +}

Yeah, no... not happening.

We spend a lot of time to never touch the old rq->lock on wakeups. Mason
was the one pushing for that, so he should very well know this.

The one cross-cpu atomic (iowait) is already a problem (the whole iowait
accounting being useless makes it even worse), adding significant remote
prodding is just really bad.