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

[Goel, Akash]

On 11/25/2015 10:58 PM, Chris Wilson wrote:
> On Wed, Nov 25, 2015 at 01:02:20PM +0200, Ville Syrjälä 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.
>>
>> Can we know that with speculative prefetching and whatnot?
>
> "The memory attribute of the page containing the affected line has no
> effect on the behavior of this instruction. It should be noted that
> processors are free to speculative fetch and cache data from system
> memory regions assigned a memory-type allowing for speculative reads
> (i.e. WB, WC, WT memory types). The Streaming SIMD Extensions PREFETCHh
> instruction is considered a hint to this speculative behavior. Because
> this speculative fetching can occur at any time and is not tied to
> instruction execution, CLFLUSH is not ordered with respect to PREFETCHh
> or any of the speculative fetching mechanisms (that is, data could be
> speculative loaded into the cache just before, during, or after the
> execution of a CLFLUSH to that cache line)."
>
> which taken to the extreme means that we can't get away with this trick.
>
> If we can at least guarantee that such speculation can't extend beyond
> a page boundary that will be enough to assert that the patch is valid.
>
> Hopefully someone knows a CPU guru or two.

Found some relevant info at the link https://lwn.net/Articles/255364/

An excerpt from the same link
Hardware Prefetching
"Prefetching has one big weakness: it cannot cross page boundaries. The 
reason should be obvious when one realizes that the CPUs support demand 
paging. If the prefetcher were allowed to cross page boundaries, the 
access might trigger an OS event to make the page available. This by 
itself can be bad, especially for performance. What is worse is that the 
prefetcher does not know about the semantics of the program or the OS 
itself. It might therefore prefetch pages which, in real life, never 
would be requested. That means the prefetcher would run past the end of 
the memory region the processor accessed in a recognizable pattern 
before. This is not only possible, it is very likely. If the processor, 
as a side effect of a prefetch, triggered a request for such a page the 
OS might even be completely thrown off its tracks if such a request 
could never otherwise happen.

It is therefore important to realize that, regardless of how good the 
prefetcher is at predicting the pattern, the program will experience 
cache misses at page boundaries unless it explicitly prefetches or reads 
from the new page. This is another reason to optimize the layout of data 
as described in Section 6.2 to minimize cache pollution by keeping 
unrelated data out.

Because of this page limitation the processors do not have terribly 
sophisticated logic to recognize prefetch patterns. With the still 
predominant 4k page size there is only so much which makes sense. The 
address range in which strides are recognized has been increased over 
the years, but it probably does not make much sense to go beyond the 512 
byte window which is often used today. Currently prefetch units do not 
recognize non-linear access pattern"


Best regards
Akash

> -Chris
>