linux-kernel - Re: [PATCH v2 2/4] drm/msm: Avoid mutex in shrinker

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Date:   Thu, 1 Apr 2021 10:37:44 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <freedreno@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>
Subject: Re: [PATCH v2 2/4] drm/msm: Avoid mutex in shrinker_count()

On Thu, Apr 1, 2021 at 8:34 AM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Wed, Mar 31, 2021 at 6:24 PM Rob Clark <robdclark@...il.com> wrote:
> >
> > @@ -45,6 +30,9 @@ msm_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
> >         list_for_each_entry(msm_obj, &priv->inactive_dontneed, mm_list) {
> >                 if (freed >= sc->nr_to_scan)
> >                         break;
> > +               /* Use trylock, because we cannot block on a obj that
> > +                * might be trying to acquire mm_lock
> > +                */
>
> nit: I thought the above multi-line commenting style was only for
> "net" subsystem?

we do use the "net" style a fair bit already.. (OTOH I tend to not
really care what checkpatch says)

> >                 if (!msm_gem_trylock(&msm_obj->base))
> >                         continue;
> >                 if (is_purgeable(msm_obj)) {
> > @@ -56,8 +44,11 @@ msm_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
> >
> >         mutex_unlock(&priv->mm_lock);
> >
> > -       if (freed > 0)
> > +       if (freed > 0) {
> >                 trace_msm_gem_purge(freed << PAGE_SHIFT);
> > +       } else {
> > +               return SHRINK_STOP;
> > +       }
>
> It probably doesn't matter, but I wonder if we should still be
> returning SHRINK_STOP if we got any trylock failures. It could
> possibly be worth returning 0 in that case?

On the surface, you'd think that, but there be mm dragons.. we can hit
shrinker from the submit path when the obj is locked already and we
are trying to allocate backing pages.  We don't want to tell vmscan to
keep trying, because we'll keep failing to grab that objects lock

>
> > @@ -75,6 +66,9 @@ vmap_shrink(struct list_head *mm_list)
> >         unsigned unmapped = 0;
> >
> >         list_for_each_entry(msm_obj, mm_list, mm_list) {
> > +               /* Use trylock, because we cannot block on a obj that
> > +                * might be trying to acquire mm_lock
> > +                */
>
> If you end up changing the commenting style above, should also be here.
>
> At this point this seems fine to land to me. Though I'm not an expert
> on every interaction in this code, I've spent enough time starting at
> it that I'm comfortable with:
>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>

thanks

BR,
-R