Re: [RFC PATCH 0/3] CFS idle injection

From: Jacob Pan
Date: Mon Nov 09 2015 - 09:43:32 EST

On Mon, 9 Nov 2015 15:15:34 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Mon, Nov 09, 2015 at 11:56:51AM +0000, Punit Agrawal wrote:
> > Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx> writes:
> > > My take is that RT and throttling will never go well together
> > > since they are conflicting in principle.
> >
> > I am not sure I follow. If RT (or other higher priority classes)
> > can't be throttled then the CPUs are not able to contribute towards
> > constraining power consumption and hence temperature.
> >
> > This is especially true in certain platforms where tasks belong to
> > the RT class to maintain user experience, e.g., audio and video.
> Audio/Video playback generally doesn't take a _lot_ of time these
> days. What is important though is _when_ it happens.
> And media playback typically already has a very well defined and
> stable cadence (24Hz or whatnot). What you want is for your idle
> injector to sync up with that, not disrupt it.
Agreed, i have tested idle injection on video playback (mostly one cpu
busy, no sync with gpu), it does not do well to improve energy
efficiency. With the video playback being offloaded, there is no
thermal condition either. So outside the scope of this first patchst
trying to solve. The ability to sync with external pattern, could be the
next step. kind of like pll in hw :).

> For other workloads, missing a deadline is about as bad as destroying
> the chip, complete system shutdown might be safer than getting
> delayed. (The very tired scenario of a saw, a laser and your finger;
> you want to shut down the entire machine rather than just cut off
> your finger.)
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