Re: [PATCH 1/2] drm/i915: Break busywaiting for requests on pending signals

From: Chris Wilson
Date: Mon Nov 16 2015 - 06:22:54 EST

On Mon, Nov 16, 2015 at 09:54:10AM +0000, Tvrtko Ursulin wrote:
> Hi,
> On 15/11/15 13:32, Chris Wilson wrote:
> >The busywait in __i915_spin_request() does not respect pending signals
> >and so may consume the entire timeslice for the task instead of
> >returning to userspace to handle the signal.
> Obviously correct to break the spin, but if spending a jiffie to
> react to signals was the only problem then it is not too severe.
> Add something to the commit message about how it was found/reported
> and about the severity of impact, etc?


At the worst case this could cause a delay in signal processing of 20ms,
which would be a noticeable jitter in cursor tracking. If a higher
resolution signal was being used, for example to provide fairness of a
server timeslices between clients, we could expect to detect some
unfairness between clients. This issue was noticed when inspecting a
report of poor interactivity resulting from excessively high
__i915_spin_request usage.

Chris Wilson, Intel Open Source Technology Centre
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