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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 09, 2018 at 12:46:18PM +0200, Peter Zijlstra wrote:
> 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.

Also, since all you need is the global number, I don't think you
actually need any of this. See what we do for nr_uninterruptible.

In general I think you want to (re)read loadavg.c some more, and maybe
reuse a bit more of that.