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

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Message-ID: <CANpmjNNtT1WUpJu_n5x_tA2sL4+utP0a6oGUzqrU5JuEu3mowg@mail.gmail.com>
Date:   Tue, 5 Sep 2023 18:19:04 +0200
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 Mon, 4 Sept 2023 at 20:46, Andrey Konovalov <andreyknvl@...il.com> wrote:
>
> On Wed, Aug 30, 2023 at 11:13 AM Marco Elver <elver@...gle.com> wrote:
> >
> > > -static int new_pool_required = 1;
> > > +static bool new_pool_required = true;
> > > +/* Lock that protects the variables above. */
> > > +static DEFINE_RWLOCK(pool_rwlock);
> >
> > Despite this being a rwlock, it'll introduce tons of (cache) contention
> > for the common case (stack depot entry exists).
> >
> > If creating new stack depot entries is only common during "warm-up" and
> > then becomes exceedingly rare, I think a percpu-rwsem (read-lock is a
> > CPU-local access, but write-locking is expensive) may be preferable.
>
> 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.