Re: [PATCH v2 2/3] drm/i915: Limit the busy wait on requests to 10us not 10ms!

From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>, intel-gfx@lists.freedesktop.org
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>,
	Eero Tamminen <eero.t.tamminen@intel.com>,
	"Rantala, Valtteri" <valtteri.rantala@intel.com>,
	stable@kernel.vger.org
Subject: Re: [PATCH v2 2/3] drm/i915: Limit the busy wait on requests to 10us not 10ms!
Date: Fri, 27 Nov 2015 16:52:58 +0000	[thread overview]
Message-ID: <56588A6A.8060400@linux.intel.com> (raw)
In-Reply-To: <1448634382-8987-2-git-send-email-chris@chris-wilson.co.uk>


On 27/11/15 14:26, 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@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

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko
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