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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 02/05/2019 15:00, Chris Wilson wrote:
> 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@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.

Okay.. lets see if IGT is playing ball. :)

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko
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