[Intel-gfx] [PATCH 07/14] drm/i915: Stop spinning for DROP_IDLE (debugfs/i915_drop_caches)

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-05-02 14:34:11)
> 
> On 01/05/2019 12:45, Chris Wilson wrote:
> > If the user is racing a call to debugfs/i915_drop_caches with ongoing
> > submission from another thread/process, we may never end up idling the
> > GPU and be uninterruptibly spinning in debugfs/i915_drop_caches trying
> > to catch an idle moment.
> > 
> > Just flush the work once, that should be enough to park the system under
> > correct conditions. Outside of those we either have a driver bug or the
> > user is racing themselves. Sadly, because the user may be provoking the
> > unwanted situation we can't put a warn here to attract attention to a
> > probable bug.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >   drivers/gpu/drm/i915/i915_debugfs.c | 4 +---
> >   1 file changed, 1 insertion(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> > index 7e8898d0b78b..2ecefacb1e66 100644
> > --- a/drivers/gpu/drm/i915/i915_debugfs.c
> > +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> > @@ -3933,9 +3933,7 @@ i915_drop_caches_set(void *data, u64 val)
> >       fs_reclaim_release(GFP_KERNEL);
> >   
> >       if (val & DROP_IDLE) {
> > -             do {
> > -                     flush_delayed_work(&i915->gem.retire_work);
> > -             } while (READ_ONCE(i915->gt.awake));
> > +             flush_delayed_work(&i915->gem.retire_work);
> >               flush_work(&i915->gem.idle_work);
> >       }
> >   
> > 
> 
> What were supposed to be semantics of DROP_IDLE? Now it seems rather 
> weak. Should it for instance also imply DROP_ACTIVE?

All I need for DROP_IDLE is that the idle worker is flushed. I've always
assumed you would pass in DROP_ACTIVE | DROP_RETIRE | DROP_IDLE as the
trifecta.

The biggest problem here is that's it is an uninterruptible loop.
-Chris