linux-kernel - Re: [PATCH 11/15] stackdepot: use read/write lock

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Date: Tue, 2 Jan 2024 13:59:06 +0100
From: Marco Elver <elver@...gle.com>
To: Andrey Konovalov <andreyknvl@...il.com>
Cc: andrey.konovalov@...ux.dev, Alexander Potapenko <glider@...gle.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com, 
	Evgenii Stepanov <eugenis@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH 11/15] stackdepot: use read/write lock

On Wed, 13 Sept 2023 at 19:09, Andrey Konovalov <andreyknvl@...il.com> wrote:
>
> On Tue, Sep 5, 2023 at 6:19 PM Marco Elver <elver@...gle.com> wrote:
> >
> > > Good suggestion. I propose that we keep the rwlock for now, and I'll
> > > check whether the performance is better with percpu-rwsem once I get
> > > to implementing and testing the performance changes. I'll also check
> > > whether percpu-rwsem makes sense for stack ring in tag-based KASAN
> > > modes.
> >
> > I think it's quite obvious that the percpu-rwsem is better. A simple
> > experiment is to measure the ratio of stackdepot hits vs misses. If
> > the ratio is obviously skewed towards hits, then I'd just go with the
> > percpu-rwsem.
> >
> > The performance benefit may not be measurable if you use a small system.
>
> I started looking into using percpu-rwsem, but it appears that it
> doesn't have the irqsave/irqrestore API flavor. I suspect that it
> shouldn't be hard to add it, but I'd rather not pursue this as a part
> of this series.
>
> So I still propose to keep the rwlock for now, and switch to
> percpu-rwsem later together with the other perf changes.

I may have gotten lost in the post-vacation email avalanche and missed
it: did you already send the percpu-rwsem optimization? I am a little
worried about the contention the plain rwlock introduces on big
machines.

Thanks,
-- Marco