Re: [PATCH v3] drm/i915 : Avoid superfluous invalidation of CPU cache lines

[Chris Wilson <chris@chris-wilson.co.uk>]

On Tue, Dec 01, 2015 at 03:28:28PM +0200, Ville Syrjälä wrote:
> On Tue, Dec 01, 2015 at 01:09:33PM +0000, Chris Wilson wrote:
> > On Tue, Dec 01, 2015 at 02:34:41PM +0200, Ville Syrjälä wrote:
> > > On Mon, Nov 30, 2015 at 12:41:05PM +0530, akash.goel@intel.com wrote:
> > > > @@ -3982,7 +3983,21 @@ i915_gem_object_set_to_cpu_domain(struct drm_i915_gem_object *obj, bool write)
> > > >  
> > > >  	/* Flush the CPU cache if it's still invalid. */
> > > >  	if ((obj->base.read_domains & I915_GEM_DOMAIN_CPU) == 0) {
> > > > -		i915_gem_clflush_object(obj, false);
> > > > +		/* If an object is moved out of the CPU domain following a
> > > > +		 * CPU write and before a GPU or GTT write, we will clflush
> > > > +		 * it out of the CPU cache, and mark the cache as clean.
> > > > +		 * After clflushing we know that this object cannot be in the
> > > > +		 * CPU cache, nor can it be speculatively loaded into the CPU
> > > > +		 * cache as our objects are page-aligned (& speculation cannot
> > > > +		 * cross page boundaries). Whilst this flag is set, we know
> > > > +		 * that any future access to the object's pages will miss the
> > > > +		 * stale cache and have to be serviced from main memory, i.e.
> > > > +		 * we do not need another clflush to invalidate the CPU cache
> > > > +		 * in preparing to read from the object.
> > > > +		 */
> > > > +		if (!obj->cache_clean)
> > > > +			i915_gem_clflush_object(obj, false);
> > > > +		obj->cache_clean = false;
> > > 
> > > Having the comment here talk about moving stuff out of the cpu domain
> > > made me think there's a bug here (false vs. true). But actually this
> > > code moves it into the cpu domain so it's actually fine, I wonder if
> > > there's a better place for the comment (eg. where we do set
> > > cache_clean=true)?
> > 
> > I thought it made more sense here because this is where we playing the
> > trick to avoid the clflush.
> > 
> > Hmm, would s/If an object/When the object/ and
> > s/cache_clean/cache_flushed/ suffice?
> 
> Maybe 'When the object is eventually moved out...' ?
> 
> That extra word might convey more clearly that's it's not talking
> about moving it out right now.

Hmm, the change of tense is good. Let's try that again:

When the object was moved out the CPU domain following a CPU write, we
will have flushed it out of the CPU cache (and marked the object as
cache_flushed). After the clflush, we know that this object cannot be in
the CPU cache, nor can it be speculatively loaded into the CPU cache as
our objects are page-aligned and speculation cannot cross page
boundaries. So whilst the cache_flushed flag is set, we know that any
future access to the object's pages will miss the GPU cache and have to
be serviced from main memory (where they will pick up any writes
through the GTT or by the GPU) i.e. we do not need another clflush here
and now to invalidate the CPU cache as we prepare to read from the object.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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