Re: [Intel-gfx] [PATCH] drm/i915: Unconditionally flush writes before execbuffer

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Daniel Vetter <daniel@ffwll.ch>,
	intel-gfx@lists.freedesktop.org,
	Akash Goel <akash.goel@intel.com>,
	stable@vger.kernel.org
Subject: Re: [Intel-gfx] [PATCH] drm/i915: Unconditionally flush writes before execbuffer
Date: Tue, 19 May 2015 15:41:48 +0100	[thread overview]
Message-ID: <20150519144148.GA13637@nuc-i3427.alporthouse.com> (raw)
In-Reply-To: <20150511152552.GD19022@nuc-i3427.alporthouse.com>

On Mon, May 11, 2015 at 04:25:52PM +0100, Chris Wilson wrote:
> On Mon, May 11, 2015 at 12:34:37PM +0200, Daniel Vetter wrote:
> > On Mon, May 11, 2015 at 08:51:36AM +0100, Chris Wilson wrote:
> > > With the advent of mmap(wc), we have a path to write directly into
> > > active GPU buffers. When combined with async updates (i.e. avoiding the
> > > explicit domain management along with the memory barriers and GPU
> > > stalls) we start to see the GPU read the wrong values from memory - i.e.
> > > we have insufficient memory barriers along the execbuffer path. Writes
> > > through the GTT should have been naturally serialised with execution
> > > through the GTT as well and so the impact only seems to be from the WC
> > > paths.
> > > 
> > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > Cc: Akash Goel <akash.goel@intel.com>
> > > Cc: stable@vger.kernel.org
> > 
> > Do we have a nasty igt for this? Bugzilla?
> 
> I've added igt/gem_streaming_writes.
> 
> That wmb() is not enough for !llc. Since the wmb() made piglit happy it
> is quite possible I haven't hit the same path exactly, but it's going to
> take some investigation to see if igt/gem_streaming_writes can possibly
> work on !llc.

Humbug.

Found the bug in gem_streaming_writes, even though I still think the
wmb() is strictly required, it runs fine without (presumably I haven't
managed to avoid all barriers in the execbuffer path yet). However, I
think can improve the stress by inserting extra gpu load -- that should
help make the CPU writes / GPU reads of the buffer concurrent?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre