[Intel-gfx] [PATCH] drm/i915 : Avoid superfluous invalidation of CPU cache lines

[Goel, Akash]

On 11/25/2015 3:30 PM, Daniel Vetter wrote:
> On Wed, Nov 25, 2015 at 02:57:47PM +0530, Goel, Akash wrote:
>>
>>
>> On 11/25/2015 2:51 PM, Daniel Vetter wrote:
>>> On Tue, Nov 24, 2015 at 10:39:38PM +0000, Chris Wilson wrote:
>>>> On Tue, Nov 24, 2015 at 07:14:31PM +0100, Daniel Vetter wrote:
>>>>> On Tue, Nov 24, 2015 at 12:04:06PM +0200, Ville Syrjälä wrote:
>>>>>> On Tue, Nov 24, 2015 at 03:35:24PM +0530, akash.goel at intel.com wrote:
>>>>>>> From: Akash Goel <akash.goel at intel.com>
>>>>>>>
>>>>>>> When the object is moved out of CPU read domain, the cachelines
>>>>>>> are not invalidated immediately. The invalidation is deferred till
>>>>>>> next time the object is brought back into CPU read domain.
>>>>>>> But the invalidation is done unconditionally, i.e. even for the case
>>>>>>> where the cachelines were flushed previously, when the object moved out
>>>>>>> of CPU write domain. This is avoidable and would lead to some optimization.
>>>>>>> Though this is not a hypothetical case, but is unlikely to occur often.
>>>>>>> The aim is to detect changes to the backing storage whilst the
>>>>>>> data is potentially in the CPU cache, and only clflush in those case.
>>>>>>>
>>>>>>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>>>>>>> Signed-off-by: Akash Goel <akash.goel at intel.com>
>>>>>>> ---
>>>>>>>   drivers/gpu/drm/i915/i915_drv.h | 1 +
>>>>>>>   drivers/gpu/drm/i915/i915_gem.c | 9 ++++++++-
>>>>>>>   2 files changed, 9 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
>>>>>>> index df9316f..fedb71d 100644
>>>>>>> --- a/drivers/gpu/drm/i915/i915_drv.h
>>>>>>> +++ b/drivers/gpu/drm/i915/i915_drv.h
>>>>>>> @@ -2098,6 +2098,7 @@ struct drm_i915_gem_object {
>>>>>>>   	unsigned long gt_ro:1;
>>>>>>>   	unsigned int cache_level:3;
>>>>>>>   	unsigned int cache_dirty:1;
>>>>>>> +	unsigned int cache_clean:1;
>>>>>>
>>>>>> So now we have cache_dirty and cache_clean which seems redundant,
>>>>>> except somehow cache_dirty != !cache_clean?
>>>>
>>>> Exactly, not entirely redundant. I did think something along MESI lines
>>>> would be useful, but that didn't capture the different meanings we
>>>> employ.
>>>>
>>>> cache_dirty tracks whether we have been eliding the clflush.
>>>>
>>>> cache_clean tracks whether we know the cache has been completely
>>>> clflushed.
>>>>
>>>> (cache_clean implies !cache_dirty, but
>>>> !cache_clean does not imply cache_dirty)
>>>>
>>>>> We also have read_domains & DOMAIN_CPU. Which is which?
>>>>
>>>> DOMAIN_CPU implies that the object may be in the cpu cache (modulo the
>>>> clflush elision above).
>>>>
>>>> DOMAIN_CPU implies !cache_clean
>>>>
>>>> and even
>>>>
>>>> cache_clean implies !DOMAIN_CPU
>>>>
>>>> but
>>>>
>>>> !DOMAIN_CPU does not imply cache_clean
>>>
>>> All the above should be in the kerneldoc (per-struct-member comments
>>> please) of drm_i915_gem_object. Akash, can you please amend your patch?
>>> And please make sure we do include that kerneldoc somewhere ... might need
>>> an upfront patch to do that, for just drm_i915_gem_object.
>>
>> I floated the amended patch, earlier today,
>> http://lists.freedesktop.org/archives/intel-gfx/2015-November/081194.html.
>> Please kindly check that.
>
> Already done and replied here because I think this should be lifted to
> kerneldoc for the structure itself. That's why I replied here ;-)
> -Daniel
Hi Daniel,

I think the patch to provide a kernel-doc for just the 
drm_i915_gem_object structure can be submitted independently of this 
patch. The kernel-doc part can be done as a follow up patch.

For the current patch, have added the per-struct-member comments for the 
'cache_clean' field.

Best regards
Akash

>