Re: [PATCH] drm/i915 : Avoid superfluous invalidation of CPU cache lines

["Goel, Akash" <akash.goel@intel.com>]

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@intel.com wrote:
>>>>> From: Akash Goel <akash.goel@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@chris-wilson.co.uk>
>>>>> Signed-off-by: Akash Goel <akash.goel@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.

Best regards
Akash


> -Daniel
>
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