[Intel-gfx] [PATCH] drm/i915: Kick rcu harder to free objects

[Ville Syrjälä]

On Thu, Sep 08, 2022 at 01:23:50PM +0100, Tvrtko Ursulin wrote:
> 
> On 06/09/2022 18:46, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > 
> > On gen3 the selftests are pretty much always tripping this:
> > <4> [383.822424] pci 0000:00:02.0: drm_WARN_ON(dev_priv->mm.shrink_count)
> > <4> [383.822546] WARNING: CPU: 2 PID: 3560 at drivers/gpu/drm/i915/i915_gem.c:1223 i915_gem_cleanup_early+0x96/0xb0 [i915]
> > 
> > Looks to be due to the status page object lingering on the
> > purge_list. Call synchronize_rcu() ahead of it to make more
> > sure all objects have been freed.
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_gem.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 0f49ec9d494a..5b61f7ad6473 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -1098,6 +1098,7 @@ void i915_gem_drain_freed_objects(struct drm_i915_private *i915)
> >   		flush_delayed_work(&i915->bdev.wq);
> >   		rcu_barrier();
> >   	}
> > +	synchronize_rcu();
> 
> Looks a bit suspicious that a loop would not free all but one last rcu 
> grace would. Definitely fixes the issue in your testing?

Definite is a bit hard to say with fuzzy stuff like this. But yes,
so far didn't see the warn triggering anymore. CI results show the same.

> 
> Perhaps the fact there is a cond_resched in __i915_gem_free_objects, but 
> then again free count should reflect the state and keep it looping in here..
> 
> Regards,
> 
> Tvrtko

-- 
Ville Syrjälä
Intel