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Date:   Mon, 6 Aug 2018 17:25:28 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Daniel Drake <drake@endlessm.com>,
        Vinayak Menon <vinmenon@codeaurora.org>,
        Christopher Lameter <cl@linux.com>,
        Mike Galbraith <efault@gmx.de>,
        Shakeel Butt <shakeelb@google.com>,
        Peter Enderborg <peter.enderborg@sony.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and
 IO
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On Mon, Aug 06, 2018 at 11:05:50AM -0400, Johannes Weiner wrote:
> Argh, that's right. This needs an explicit count if we want to access
> it locklessly. And you already said you didn't like that this is the
> only state not derived purely from the task counters, so maybe this is
> the way to go after all.
> 
> How about something like this (untested)?


> +static inline void psi_switch(struct rq *rq, struct task_struct *prev,
> +			      struct task_struct *next)
> +{
> +	if (psi_disabled)
> +		return;
> +
> +	if (unlikely(prev->flags & PF_MEMSTALL))
> +		psi_task_change(prev, rq_clock(rq), TSK_RECLAIMING, 0);
> +	if (unlikely(next->flags & PF_MEMSTALL))
> +		psi_task_change(next, rq_clock(rq), 0, TSK_RECLAIMING);
> +}


Urgh... can't say I really like that.

I would really rather do that scheduler_tick() thing to avoid the remote
update. The tick is a lot less hot than the switch path and esp.
next->flags might be a cold line (prev->flags is typically the same line
as prev->state so we already have that, but I don't think anybody now
looks at next->flags or its line, so that'd be cold load).