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

[Peter Zijlstra]

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.