Re: [PATCH] drm/i915: Only move to the CPU write domain if keeping the GTT pages

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Aug 06, 2015 at 05:43:39PM +0100, Chris Wilson wrote:
> We have for a long time been ultra-paranoid about the situation whereby
> we hand back pages to the system that have been written to by the GPU
> and potentially simultaneously by the user through a CPU mmapping. We
> can relax this restriction when we know that the cache domain tracking
> is true and there can be no stale cacheline invalidatations. This is
> true if the object has never been CPU mmaped as all internal accesses
> (i.e. kmap/iomap) are carefully flushed. For a CPU mmaping, one would
> expect that the invalid cache lines are resolved on PTE/TLB shootdown
> during munmap(), so the only situation we need to be paranoid about is

That seems a pretty strong assumption given that x86 cache are physically
indexed - I'd never expect flushing to happen on munmap.

> when such a CPU mmaping exists at the time of put_pages. Given that we
> need to treat put_pages carefully as we may return live data to the
> system that we want to use again in the future (i.e. I915_MADV_WILLNEED
> pages) we can simply treat a live CPU mmaping as a special case of
> WILLNEED (which it is!). Any I915_MADV_DONTNEED pages and their
> mmapings are shotdown immediately following put_pages.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: "Goel, Akash" <akash.goel@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Jesse Barnes <jbarnes@virtuousgeek.org>

But it's still salvageable I think since we only care about coherency for
the gpu (where data might be stuck in cpu caches). From the cpu's pov (and
hence the entire system except the gpu) we should never see inconsistency
really - as soon as the gpu does a write to a cacheline it'll win, and
before that nothing in the system can assume anything about the contents
of these pages.

So imo a simple

	/* For purgeable objects we don't care about object contents. */

would be enough.

Well except that there's gl extensions which expose purgeable objects, so
I guess we can't actually do this.

I presume though you only want to avoid clflush when actually purging an
object, so maybe we can keep this by purging the shmem backing node first
and checking here for __I915_MADV_PURGED instead?

Oh and some perf data would be good for this.
-Daniel

> ---
>  drivers/gpu/drm/i915/i915_gem.c | 49 ++++++++++++++++++++++++++++++-----------
>  1 file changed, 36 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 2dfe707f11d3..24deace364a5 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2047,22 +2047,45 @@ i915_gem_object_put_pages_gtt(struct drm_i915_gem_object *obj)
>  
>  	BUG_ON(obj->madv == __I915_MADV_PURGED);
>  
> -	ret = i915_gem_object_set_to_cpu_domain(obj, true);
> -	if (ret) {
> -		/* In the event of a disaster, abandon all caches and
> -		 * hope for the best.
> -		 */
> -		WARN_ON(ret != -EIO);
> -		i915_gem_clflush_object(obj, true);
> -		obj->base.read_domains = obj->base.write_domain = I915_GEM_DOMAIN_CPU;
> -	}
> -
>  	i915_gem_gtt_finish_object(obj);
>  
> -	if (i915_gem_object_needs_bit17_swizzle(obj))
> -		i915_gem_object_save_bit_17_swizzle(obj);
> +	/* If we need to access the data in the future, we need to
> +	 * be sure that the contents of the object is coherent with
> +	 * the CPU prior to releasing the pages back to the system.
> +	 * Once we unpin them, the mm is free to move them to different
> +	 * zones or even swap them out to disk - all without our
> +	 * intervention. (Though we could track such operations with our
> +	 * own gemfs, if we ever write one.) As such if we want to keep
> +	 * the data, set it to the CPU domain now just in case someone
> +	 * else touches it.
> +	 *
> +	 * For a long time we have been paranoid about handing back
> +	 * pages to the system with stale cacheline invalidation. For
> +	 * all internal use (kmap/iomap), we know that the domain tracking is
> +	 * accurate. However, the userspace API is lax and the user can CPU
> +	 * mmap the object and invalidate cachelines without our accurate
> +	 * tracking. We have been paranoid to be sure that we always flushed
> +	 * the cachelines when we stopped using the pages. However, given
> +	 * that the CPU PTE/TLB shootdown must have invalidated the cachelines
> +	 * upon munmap(), we only need to be paranoid about a live CPU mmap
> +	 * now. For this, we need only treat it as live data see
> +	 * discard_backing_storage().
> +	 */
> +	if (obj->madv == I915_MADV_WILLNEED) {
> +		ret = i915_gem_object_set_to_cpu_domain(obj, true);
> +		if (ret) {
> +			/* In the event of a disaster, abandon all caches and
> +			 * hope for the best.
> +			 */
> +			WARN_ON(ret != -EIO);
> +			i915_gem_clflush_object(obj, true);
> +			obj->base.read_domains = I915_GEM_DOMAIN_CPU;
> +			obj->base.write_domain = I915_GEM_DOMAIN_CPU;
> +		}
>  
> -	if (obj->madv == I915_MADV_DONTNEED)
> +		if (i915_gem_object_needs_bit17_swizzle(obj))
> +			i915_gem_object_save_bit_17_swizzle(obj);
> +	} else
>  		obj->dirty = 0;
>  
>  	st_for_each_page(&iter, obj->pages) {
> -- 
> 2.5.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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