[Intel-gfx] [PATCH] drm/i915: Mitigate retirement starvation a bit

[Chris Wilson]

On Thu, Feb 04, 2016 at 12:25:24PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> In execlists mode internal house keeping of the discarded
> requests (and so contexts and VMAs) relies solely on the retire
> worker, which can be prevented from running by just being
> unlucky when busy clients are hammering on the big lock.
> 
> Prime example is the gem_close_race IGT, which due to this
> effect causes internal lists to grow to epic proportions, with
> a consequece of object VMA traversal to growing exponentially
> and resulting in tens of minutes test runtime. Memory use is
> also very high and a limiting factor on some platforms.
> 
> Since we do not want to run this internal house keeping more
> frequently, due concerns that it may affect performance, and
> the scenario being statistically not very likely in real
> workloads, one possible workaround is to run it when new
> client handles are opened.
> 
> This will solve the issues with this particular test case,
> making it complete in tens of seconds instead of tens of
> minutes, and will not add any run-time penalty to running
> clients.
> 
> It can only slightly slow down new client startup, but on a
> realisticaly loaded system we are expecting this to be not
> significant. Even with heavy rendering in progress we can have
> perhaps up to several thousands of requests pending retirement,
> which, with a typical retirement cost of 80ns to 1us per
> request, is not significant.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Testcase: igt/gem_close_race/gem-close-race
> Cc: Chris Wilson <chris at chris-wilson.co.uk>

Still doesn't fix actual workloads where this is demonstrably bad, which
can be demonstrated with a single fd.

The most effective treatment I found is moving the retire-requests from
execbuf (which exists for similar reasons) to get-pages.

http://cgit.freedesktop.org/~ickle/linux-2.6/commit/?h=breadcrumbs&id=75f4e53f1c9141ba2dd8847396a1bcc8dbeecd55

I would also suggest

http://cgit.freedesktop.org/~ickle/linux-2.6/commit/?h=breadcrumbs&id=7c1b679c76524780f8e15cc8b0c6652539182d51

for the reasons above.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre