[Intel-gfx] [PATCH 02/15] drm/i915: Limit the busy wait on requests to 10us not 10ms!

[Chris Wilson]

On Mon, Nov 30, 2015 at 10:02:30AM +0000, Tvrtko Ursulin wrote:
> 
> 
> On 29/11/15 08:48, Chris Wilson wrote:
> >When waiting for high frequency requests, the finite amount of time
> >required to set up the irq and wait upon it limits the response rate. By
> >busywaiting on the request completion for a short while we can service
> >the high frequency waits as quick as possible. However, if it is a slow
> >request, we want to sleep as quickly as possible. The tradeoff between
> >waiting and sleeping is roughly the time it takes to sleep on a request,
> >on the order of a microsecond. Based on measurements of synchronous
> >workloads from across big core and little atom, I have set the limit for
> >busywaiting as 10 microseconds. In most of the synchronous cases, we can
> >reduce the limit down to as little as 2 miscroseconds, but that leaves
> >quite a few test cases regressing by factors of 3 and more.
> >
> >The code currently uses the jiffie clock, but that is far too coarse (on
> >the order of 10 milliseconds) and results in poor interactivity as the
> >CPU ends up being hogged by slow requests. To get microsecond resolution
> >we need to use a high resolution timer. The cheapest of which is polling
> >local_clock(), but that is only valid on the same CPU. If we switch CPUs
> >because the task was preempted, we can also use that as an indicator that
> >  the system is too busy to waste cycles on spinning and we should sleep
> >instead.
> >
> >__i915_spin_request was introduced in
> >commit 2def4ad99befa25775dd2f714fdd4d92faec6e34 [v4.2]
> >Author: Chris Wilson <chris at chris-wilson.co.uk>
> >Date:   Tue Apr 7 16:20:41 2015 +0100
> >
> >      drm/i915: Optimistically spin for the request completion
> >
> >v2: Drop full u64 for unsigned long - the timer is 32bit wraparound safe,
> >so we can use native register sizes on smaller architectures. Mention
> >the approximate microseconds units for elapsed time and add some extra
> >comments describing the reason for busywaiting.
> >
> >v3: Raise the limit to 10us
> >
> >Reported-by: Jens Axboe <axboe at kernel.dk>
> >Link: https://lkml.org/lkml/2015/11/12/621
> >Cc; "Rogozhkin, Dmitry V" <dmitry.v.rogozhkin at intel.com>
> >Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> >Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> >Cc: Eero Tamminen <eero.t.tamminen at intel.com>
> >Cc: "Rantala, Valtteri" <valtteri.rantala at intel.com>
> >Cc: stable at kernel.vger.org
> >---
> >  drivers/gpu/drm/i915/i915_gem.c | 47 +++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 45 insertions(+), 2 deletions(-)
> 
> Again,
> 
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

Sorry, I was just including these patches here so that anyone who wanted
to look at the wait-request patches only had one series to pull :)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre