linux-kernel - Re: [PATCH v6 17/22] drm/shmem-helper: Add generic memory shrinker

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Date:   Fri, 24 Jun 2022 22:21:52 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Rob Clark <robdclark@...il.com>
Cc:     Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        David Airlie <airlied@...ux.ie>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Daniel Almeida <daniel.almeida@...labora.com>,
        Gert Wollny <gert.wollny@...labora.com>,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Daniel Stone <daniel@...ishbar.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Rob Herring <robh@...nel.org>,
        Steven Price <steven.price@....com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Qiang Yu <yuq825@...il.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Dmitry Osipenko <digetx@...il.com>,
        linux-tegra@...r.kernel.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, amd-gfx@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, kernel@...labora.com
Subject: Re: [PATCH v6 17/22] drm/shmem-helper: Add generic memory shrinker

On Sun, Jun 19, 2022 at 10:53:03AM -0700, Rob Clark wrote:
> On Thu, May 26, 2022 at 4:55 PM Dmitry Osipenko
> <dmitry.osipenko@...labora.com> wrote:
> > +       mutex_unlock(&gem_shrinker->lock);
> 
> As I mentioned on other thread, count_objects, being approximate but
> lockless and fast is the important thing.  Otherwise when you start
> hitting the shrinker on many threads, you end up serializing them all,
> even if you have no pages to return to the system at that point.

Yeah agreed, seems like I was wrong here :-) Atomic counter or something
would also be in link the the lru_list stuff.

It would be to record this in the kerneldoc for the shrinker structure
though, to make sure this is all understood.

> > +               /* prevent racing with the dma-buf importing/exporting */
> > +               if (!mutex_trylock(&gem_shrinker->dev->object_name_lock)) {
> > +                       *lock_contention |= true;
> > +                       goto resv_unlock;
> > +               }
> 
> I'm not sure this is a good idea to serialize on object_name_lock.
> Purgeable buffers should never be shared (imported or exported).  So
> at best you are avoiding evicting and immediately swapping back in, in
> a rare case, at the cost of serializing multiple threads trying to
> reclaim pages in parallel.

Yeah this sounds really bad. Plus this is a per-device lock, and doing
those with trylock means the shrinker will fail to find shrinkable memory
way too often. We need to engineer this out somehow.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch