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

[Chris Wilson]

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
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre