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@xxxxxxxxxxxxxxxxxx> > >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@xxxxxxxxx> > >Link: https://lkml.org/lkml/2015/11/12/621 > >Cc; "Rogozhkin, Dmitry V" <dmitry.v.rogozhkin@xxxxxxxxx> > >Cc: Daniel Vetter <daniel.vetter@xxxxxxxx> > >Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> > >Cc: Eero Tamminen <eero.t.tamminen@xxxxxxxxx> > >Cc: "Rantala, Valtteri" <valtteri.rantala@xxxxxxxxx> > >Cc: stable@xxxxxxxxxxxxxxx > >--- > > drivers/gpu/drm/i915/i915_gem.c | 47 +++++++++++++++++++++++++++++++++++++++-- > > 1 file changed, 45 insertions(+), 2 deletions(-) > > Again, > > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx> 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 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
