Re: [PATCH 13/14] drm/msm: Drop struct_mutex in shrinker path

[Rob Clark]

On Mon, Oct 5, 2020 at 5:44 PM Hillf Danton <hdanton@xxxxxxxx> wrote:
>
>
> On Mon, 5 Oct 2020 18:17:01 Kristian H. Kristensen wrote:
> > On Mon, Oct 5, 2020 at 4:02 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > >
> > > On Mon, Oct 05, 2020 at 05:24:19PM +0800, Hillf Danton wrote:
> > > >
> > > > On Sun,  4 Oct 2020 12:21:45
> > > > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > > >
> > > > > Now that the inactive_list is protected by mm_lock, and everything
> > > > > else on per-obj basis is protected by obj->lock, we no longer depend
> > > > > on struct_mutex.
> > > > >
> > > > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > > > ---
> > > > >  drivers/gpu/drm/msm/msm_gem.c          |  1 -
> > > > >  drivers/gpu/drm/msm/msm_gem_shrinker.c | 54 --------------------------
> > > > >  2 files changed, 55 deletions(-)
> > > > >
> > > > [...]
> > > >
> > > > > @@ -71,13 +33,8 @@ msm_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
> > > > >  {
> > > > >     struct msm_drm_private *priv =
> > > > >             container_of(shrinker, struct msm_drm_private, shrinker);
> > > > > -   struct drm_device *dev = priv->dev;
> > > > >     struct msm_gem_object *msm_obj;
> > > > >     unsigned long freed = 0;
> > > > > -   bool unlock;
> > > > > -
> > > > > -   if (!msm_gem_shrinker_lock(dev, &unlock))
> > > > > -           return SHRINK_STOP;
> > > > >
> > > > >     mutex_lock(&priv->mm_lock);
> > > >
> > > > Better if the change in behavior is documented that SHRINK_STOP will
> > > > no longer be needed.
> > >
> > > btw I read through this and noticed you have your own obj lock, plus
> > > mutex_lock_nested. I strongly recommend to just cut over to dma_resv_lock
> > > for all object lock needs (soc drivers have been terrible with this
> > > unfortuntaly), and in the shrinker just use dma_resv_trylock instead of
> > > trying to play clever games outsmarting lockdep.
>
> The trylock makes page reclaimers turn to their next target e.g. inode
> cache instead of waiting for the mutex to be released. It makes sense
> for instance in scenarios of mild memory pressure.

is there some behind-the-scenes signalling for this, or is this just
down to what the shrinker callbacks return?  Generally when we get
into shrinking, there are a big set of purgable bo's to consider, so
the shrinker callback return wouldn't be considering just one
potentially lock contended bo (buffer object).  Ie failing one
trylock, we just move on to the next.

fwiw, what I've seen on the userspace bo cache vs shrinker (anything
that is shrinker potential is in userspace bo cache and
MADV(WONTNEED)) is that in steady state I see a very strong recycling
of bo's (which avoids allocating and mmap'ing or mapping to gpu a new
buffer object), so it is definitely a win in mmap/realloc bandwidth..
in steady state there is a lot of free and realloc of same-sized
buffers from frame to frame.

But in transient situations like moving to new game level when there
is a heavy memory pressure and lots of freeing old
buffers/textures/etc and then allocating new ones, I see shrinker
kicking in hard (in android situations, not so much so with
traditional linux userspace)

BR,
-R

>
> > >
> > > I recently wrote an entire blog length rant on why I think
> > > mutex_lock_nested is too dangerous to be useful:
> > >
> > > https://blog.ffwll.ch/2020/08/lockdep-false-positives.html
> > >
> > > Not anything about this here, just general comment. The problem extends to
> > > shmem helpers and all that also having their own locks for everything.
> >
> > This is definitely a tangible improvement though - very happy to see
> > msm_gem_shrinker_lock() go.
> >
> > Reviewed-by: Kristian H. Kristensen <hoegsberg@xxxxxxxxxx>
> >
> > > -Daniel
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
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>