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From: Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH v2 5/5] psi: introduce psi monitor
Date: Wed, 16 Jan 2019 14:17:28 -0500
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To: Suren Baghdasaryan <surenb@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Tejun Heo <tj@kernel.org>, lizefan@huawei.com, axboe@kernel.dk, dennis@kernel.org, Dennis Zhou <dennisszhou@gmail.com>, Ingo Molnar <mingo@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Jonathan Corbet <corbet@lwn.net>, cgroups@vger.kernel.org, linux-mm <linux-mm@kvack.org>, linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>, kernel-team@android.com

On Wed, Jan 16, 2019 at 09:39:13AM -0800, Suren Baghdasaryan wrote:
> On Wed, Jan 16, 2019 at 5:24 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, Jan 14, 2019 at 11:30:12AM -0800, Suren Baghdasaryan wrote:
> > > For memory ordering (which Johannes also pointed out) the critical point is:
> > >
> > > times[cpu] += delta           | if g->polling:
> > > smp_wmb()                     |   g->polling = polling = 0
> > > cmpxchg(g->polling, 0, 1)     |   smp_rmb()
> > >                               |   delta = times[*] (through goto SLOWPATH)
> > >
> > > So that hotpath writes to times[] then g->polling and slowpath reads
> > > g->polling then times[]. cmpxchg() implies a full barrier, so we can
> > > drop smp_wmb(). Something like this:
> > >
> > > times[cpu] += delta           | if g->polling:
> > > cmpxchg(g->polling, 0, 1)     |   g->polling = polling = 0
> > >                               |   smp_rmb()
> > >                               |   delta = times[*] (through goto SLOWPATH)
> > >
> > > Would that address your concern about ordering?
> >
> > cmpxchg() implies smp_mb() before and after, so the smp_wmb() on the
> > left column is superfluous.
> 
> Should I keep it in the comments to make it obvious and add a note
> about implicit barriers being the reason we don't call smp_mb() in the
> code explicitly?

I'd keep 'em out if they aren't actually in the code. But I'd switch

	delta = times[*]

in this comment to to

	get_recent_times() // implies smp_mb()

or something to make the ordering a bit more visible.

And also add a comment to the actual cmpxchg() in the code directly
that says that we rely on the implied ordering and that it pairs with
the smp_mb() in the slowpath; add a similar comment to the smp_mb().

> > Also, you probably want to use atomic_t for g->polling, because we
> > (sadly) have architectures where regular stores and atomic ops don't
> > work 'right'.
> 
> Oh, I see. Will do. Thanks!

Yikes, that's news to me too. Good to know.