From: Johannes Weiner <hannes@cmpxchg.org>
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
Subject: Re: [PATCH v2 5/5] psi: introduce psi monitor
Date: Wed, 16 Jan 2019 14:17:28 -0500 [thread overview]
Message-ID: <20190116191728.GA1380@cmpxchg.org> (raw)
In-Reply-To: <CAJuCfpEJW6Uq4GSGEGLKOM4K7ySHUeTGrSUGM1+EJSQ16d8SJg@mail.gmail.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.
next prev parent reply other threads:[~2019-01-16 19:17 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-10 22:07 [PATCH v2 0/5] psi: pressure stall monitors v2 Suren Baghdasaryan
2019-01-10 22:07 ` [PATCH v2 1/5] fs: kernfs: add poll file operation Suren Baghdasaryan
2019-01-11 6:25 ` Greg KH
2019-01-10 22:07 ` [PATCH v2 2/5] kernel: cgroup: " Suren Baghdasaryan
2019-01-10 22:07 ` [PATCH v2 3/5] psi: introduce state_mask to represent stalled psi states Suren Baghdasaryan
2019-01-10 22:07 ` [PATCH v2 4/5] psi: rename psi fields in preparation for psi trigger addition Suren Baghdasaryan
2019-01-10 22:07 ` [PATCH v2 5/5] psi: introduce psi monitor Suren Baghdasaryan
2019-01-14 10:21 ` Peter Zijlstra
2019-01-14 19:30 ` Suren Baghdasaryan
2019-01-14 19:42 ` Johannes Weiner
2019-01-14 20:47 ` Suren Baghdasaryan
2019-01-16 13:24 ` Peter Zijlstra
2019-01-16 17:39 ` Suren Baghdasaryan
2019-01-16 19:17 ` Johannes Weiner [this message]
2019-01-16 19:27 ` Johannes Weiner
2019-01-16 21:29 ` Suren Baghdasaryan
2019-01-17 8:13 ` Peter Zijlstra
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