linux-kernel - Re: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO

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Date:   Sat, 14 Jul 2018 11:02:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Mike Galbraith <efault@....de>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 08/10] psi: pressure stall information for CPU, memory,
 and IO

On Fri, Jul 13, 2018 at 12:17:56PM -0400, Johannes Weiner wrote:
> On Fri, Jul 13, 2018 at 11:21:53AM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 12, 2018 at 01:29:40PM -0400, Johannes Weiner wrote:
> > > +static inline void psi_ttwu_dequeue(struct task_struct *p)
> > > +{
> > > +	if (psi_disabled)
> > > +		return;
> > > +	/*
> > > +	 * 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);
> > > +	}
> > > +}
> > 
> > Still NAK, what happened to this here:

> That's my thought process, anyway. I'd be more than happy to make this
> more lightweight, but I don't see a way to do it without losing
> significant functional precision.

I think you're going to have to. We put a lot of effort into not taking
the old rq->lock on remote wakeups and got a significant performance
benefit from that.

You just utterly destroyed that for workloads with a high number of
iowait wakeups.