Re: [PATCH 2/4] drm/msm: Avoid mutex in shrinker_count()

Doug Anderson
2021-03-31T23:40:46Z

Hi,

On Wed, Mar 31, 2021 at 4:23 PM Rob Clark <robdclark@gmail.com> wrote:
>
> On Wed, Mar 31, 2021 at 3:44 PM Doug Anderson <dianders@chromium.org> wrote:
> >
> > Hi,
> >
> > On Wed, Mar 31, 2021 at 3:14 PM Rob Clark <robdclark@gmail.com> wrote:
> > >
> > > @@ -818,11 +820,19 @@ static void update_inactive(struct msm_gem_object *msm_obj)
> > > mutex_lock(&priv->mm_lock);
> > > WARN_ON(msm_obj->active_count != 0);
> > >
> > > + if (msm_obj->dontneed)
> > > + mark_unpurgable(msm_obj);
> > > +
> > > list_del_init(&msm_obj->mm_list);
> > > - if (msm_obj->madv == MSM_MADV_WILLNEED)
> > > + if (msm_obj->madv == MSM_MADV_WILLNEED) {
> > > list_add_tail(&msm_obj->mm_list, &priv->inactive_willneed);
> > > - else
> > > + } else if (msm_obj->madv == MSM_MADV_DONTNEED) {
> > > list_add_tail(&msm_obj->mm_list, &priv->inactive_dontneed);
> > > + mark_purgable(msm_obj);
> > > + } else {
> > > + WARN_ON(msm_obj->madv != __MSM_MADV_PURGED);
> > > + list_add_tail(&msm_obj->mm_list, &priv->inactive_purged);
> >
> > I'm probably being dense, but what's the point of adding it to the
> > "inactive_purged" list here? You never look at that list, right? You
> > already did a list_del_init() on this object's list pointer
> > ("mm_list"). I don't see how adding it to a bogus list helps with
> > anything.
>
> It preserves the "every bo is in one of these lists" statement, but
> other than that you are right we aren't otherwise doing anything with
> that list. (Or we could replace the list_del_init() with list_del()..
> I tend to instinctively go for list_del_init())

If you really want this list, it wouldn't hurt to at least have a
comment saying that it's not used for anything so people like me doing
go trying to figure out what it's used for. ;-)

> > > @@ -198,6 +203,33 @@ static inline bool is_vunmapable(struct msm_gem_object *msm_obj)
> > > return (msm_obj->vmap_count == 0) && msm_obj->vaddr;
> > > }
> > >
> > > +static inline void mark_purgable(struct msm_gem_object *msm_obj)
> > > +{
> > > + struct msm_drm_private *priv = msm_obj->base.dev->dev_private;
> > > +
> > > + WARN_ON(!mutex_is_locked(&priv->mm_lock));
> > > +
> > > + if (WARN_ON(msm_obj->dontneed))
> > > + return;
> >
> > The is_purgeable() function also checks other things besides just
> > "MSM_MADV_DONTNEED". Do we need to check those too? Specifically:
> >
> > msm_obj->sgt && !msm_obj->base.dma_buf && !msm_obj->base.import_attach
> >
> > ...or is it just being paranoid?
> >
> > I guess I'm just worried that if any of those might be important then
> > we'll consistently report back that we have a count of things that can
> > be purged but then scan() won't find anything to do. That wouldn't be
> > great.
>
> Hmm, I thought msm_gem_madvise() returned an error instead of allowing
> MSM_MADV_DONTNEED to be set on imported/exported dma-bufs.. it
> probably should to be complete (but userspace already knows not to
> madvise an imported/exported buffer for other reasons.. ie. we can't
> let a shared buffer end up in the bo cache). I'll re-work that a bit.
>
> The msm_obj->sgt case is a bit more tricky.. that will be the case of
> a freshly allocated obj that does not have backing patches yet. But
> it seems like enough of a corner case, that I'm happy to live with
> it.. ie. the tricky thing is not leaking decrements of
> priv->shrinkable_count or underflowing priv->shrinkable_count, and
> caring about the !msm_obj->sgt case doubles the number of states an
> object can be in, and the shrinker->count() return value is just an
> estimate.

I think it's equally important to make sure that we don't constantly
have a non-zero count and then have scan() do nothing. If there's a
transitory blip then it's fine, but it's not OK if it can be steady
state. Then you end up with:

1. How many objects do you have to free? 10
2. OK, free some. How many did you free? 0
3. Oh. You got more to do, I'll call you again.
4. Goto #1

...and it just keeps looping, right?

As long as you're confident that this case can't happen then we're
probably fine, but good to be careful. Is there any way we can make
sure that a "freshly allocated object" isn't ever in the "DONTNEED"
state?

> > > + priv->shrinkable_count += msm_obj->base.size >> PAGE_SHIFT;
> > > + msm_obj->dontneed = true;
> > > +}
> > > +
> > > +static inline void mark_unpurgable(struct msm_gem_object *msm_obj)
> > > +{
> > > + struct msm_drm_private *priv = msm_obj->base.dev->dev_private;
> > > +
> > > + WARN_ON(!mutex_is_locked(&priv->mm_lock));
> > > +
> > > + if (WARN_ON(!msm_obj->dontneed))
> > > + return;
> > > +
> > > + priv->shrinkable_count -= msm_obj->base.size >> PAGE_SHIFT;
> > > + WARN_ON(priv->shrinkable_count < 0);
> >
> > If you changed the order maybe you could make shrinkable_count
> > "unsigned long" to match the shrinker API?
> >
> > new_shrinkable = msm_obj->base.size >> PAGE_SHIFT;
> > WARN_ON(new_shrinkable > priv->shrinkable_count);
> > priv->shrinkable_count -= new_shrinkable
> >
>
> True, although I've developed a preference for signed integers in
> cases where it can underflow if you mess up

Yeah, I guess it's fine since it's a count of pages and we really
can't have _that_ many pages worth of stuff to purge. It might not
hurt to at least declare it as a "long" instead of an "int" though to
match the shrinker API.

-Doug