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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Nov 30, 2015 at 12:41:05PM +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.
> 
> v2: Made the comment more verbose (Ville/Chris)
>     Added doc for 'cache_clean' field (Daniel)
> 
> v3: Updated the comment to assuage an apprehension regarding the
>     speculative-prefetching behavior of HW (Ville/Chris)
> 
> Testcase: igt/gem_concurrent_blit
> Testcase: igt/benchmarks/gem_set_domain
> 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 |  9 +++++++++
>  drivers/gpu/drm/i915/i915_gem.c | 17 ++++++++++++++++-
>  2 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 11ae5a5..f97795e 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2100,6 +2100,15 @@ struct drm_i915_gem_object {
>  	unsigned int cache_level:3;
>  	unsigned int cache_dirty:1;
>  
> +	/*
> +	 * Tracks if the CPU cache has been completely clflushed.
> +	 * !cache_clean does not imply cache_dirty (there is some data in the
> +	 * CPU cachelines, but has not been dirtied), but cache_clean
> +	 * does imply !cache_dirty (no data in cachelines, so not dirty also).
> +	 * Actually cache_dirty tracks whether we have been omitting clflushes.
> +	 */
> +	unsigned int cache_clean:1;

Maybe it should be cache_flushed or something? clean really makes me
think !dirty.

> +
>  	unsigned int frontbuffer_bits:INTEL_FRONTBUFFER_BITS;
>  
>  	unsigned int pin_display;
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 33adc8f..7376be8 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3552,6 +3552,7 @@ i915_gem_clflush_object(struct drm_i915_gem_object *obj,
>  	trace_i915_gem_object_clflush(obj);
>  	drm_clflush_sg(obj->pages);
>  	obj->cache_dirty = false;
> +	obj->cache_clean = true;
>  
>  	return true;
>  }
> @@ -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)?

>  
>  		obj->base.read_domains |= I915_GEM_DOMAIN_CPU;
>  	}
> -- 
> 1.9.2

-- 
Ville Syrjälä
Intel OTC
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