Re: [RFC] drm/i915: Temporarily go realtime when polling PCODE

From: Imre Deak <imre.deak@intel.com>
To: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Intel-gfx@lists.freedesktop.org
Subject: Re: [RFC] drm/i915: Temporarily go realtime when polling PCODE
Date: Wed, 22 Feb 2017 11:13:31 +0200	[thread overview]
Message-ID: <20170222091331.GB30330@ideak-desk.fi.intel.com> (raw)
In-Reply-To: <06bc9f55-9690-e557-4f93-a119b9bb1add@linux.intel.com>

On Wed, Feb 22, 2017 at 07:52:01AM +0000, Tvrtko Ursulin wrote:
> 
> On 21/02/2017 18:48, Imre Deak wrote:
> >On Tue, Feb 21, 2017 at 05:01:58PM +0000, Tvrtko Ursulin wrote:
> >>From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >>
> >>Elevate task scheduling policy to realtime when polling on PCODE
> >>to guarantee a good poll rate before falling back to busy wait.
> >>
> >>We only do this for tasks with normal policy and priority in
> >>order  to simplify policy restore and also assuming that for
> >>tasks which either made themselves low or high priority it makes
> >>less sense to do so.
> >>
> >>Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >>Cc: Imre Deak <imre.deak@intel.com>
> >>Cc: Chris Wilson <chris@chris-wilson.co.uk>
> >>---
> >>This was my idea as mentioned in the other thread.
> >>
> >>Deadline scheduling policy seems trickier to restore from so
> >>I thought SCHED_FIFO should be good enough.
> >>
> >>Briefly tested but couldn't reproduce the timeout condition.
> >
> >Hm, I thought you wanted this instead of the preempt-disable poll. The
> >first preempt-enable poll is what's based on the spec, which only
> >requires two requests 3ms apart, so no requirement on the number of
> >requests there. That works most of the time and the preempt-disable part
> >is needed only rarely. So do we want to increase the priority for the
> >normal case?
> 
> So we end up in the busy loop case less often or never? (By polling better
> in the sleeping loop.) It is possible I got this completely wrong mind you.
> I was just going by what is written in this thread - that the problem is the
> sleeping loop sometimes does not run the COND often enough, or enough times.

Yes, but that means we also raise the priority for the usual case. That
would make the first loop a similar busy loop to what we want to avoid,
running that always. What I hope is that this is a problem in the PCODE
firmware that will get solved eventually, so we don't need the WA; hence
argued about keeping any WA separate.

--Imre
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