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

From	Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 1/2] drm/i915: Break busywaiting for requests on pending signals
Date	2015-11-16 12:50 +0100
Message-ID	<qvqsi-6MZ-27@gated-at.bofh.it> (permalink)
References	<qv5Hc-20o-3@gated-at.bofh.it> <qvoJQ-5Gr-7@gated-at.bofh.it> <qvq8W-6FI-1@gated-at.bofh.it>
Organization	Intel Corporation UK Plc

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On 16/11/15 11:22, Chris Wilson wrote:
> 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?
>
> Perhaps:
>
> 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.

Oh its the Xorg scheduler tick... I always forget about that. Was 
thinking that it is only about fatal, or at least infrequent signals.

Regards,

Tvrtko
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[PATCH 1/2] drm/i915: Break busywaiting for requests on pending signals Chris Wilson <chris@chris-wilson.co.uk> - 2015-11-15 14:40 +0100
  Re: [PATCH 1/2] drm/i915: Break busywaiting for requests on pending  signals Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> - 2015-11-16 11:00 +0100
    Re: [PATCH 1/2] drm/i915: Break busywaiting for requests on pending  signals Chris Wilson <chris@chris-wilson.co.uk> - 2015-11-16 12:30 +0100
      Re: [PATCH 1/2] drm/i915: Break busywaiting for requests on pending  signals Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> - 2015-11-16 12:50 +0100

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