linux-kernel - Re: [PATCH] timers: Fix timer inaccuracy

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Date:   Sat, 12 Nov 2016 12:25:07 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Saravana Kannan <skannan@...eaurora.org>
cc:     Joonwoo Park <joonwoop@...eaurora.org>,
        John Stultz <john.stultz@...aro.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH] timers: Fix timer inaccuracy

On Fri, 11 Nov 2016, Saravana Kannan wrote:
> On 11/10/2016 02:07 AM, Thomas Gleixner wrote:
> > Deferrable timers shouldn't have been invented in the first place and yes,
> > they are not going to happen on hrtimers, quite the contrary, I'm working
> > on eliminating them completely.
> 
> If you do that, how exactly do you propose drivers do periodic polling while
> the Linux isn't idling, but stop polling when Linux is idle? devfreq is a
> classic example. devfreq is used in a lot of mobile devices. Across different
> vendors, devfreq is used for scaling system memory, flash storage, GPU, etc.
> You are going to kill power if you remove deferrable timers without having an
> alternate mechanism to solve this requirement.
> 
> For example, when you are browsing on your phone and reading something on
> screen (but not interacting with the device), the CPUs/clusters/caches go idle
> for long periods (several seconds) of time. If you remove deferrable timers,
> you are going to force a CPU wake up every 10ms or 50ms or whatever it's
> configured for.

I know how all that works, but there are other ways to deal with those
timers. We had complaints in the past because those timers can be stuck
forever on a fully idle cpu and that's not a good thing either. The whole
concept is ill defined or not defined at all, lacks any form of sane
semantics and was shoved into the kernel w/i much thought. In hindsight I
should have rejected that deferrable mess 5+ years ago.
 
> > When you can come up with a real regression caused by the rework and not
> > just some handwaving theories then we can revisit that, but until then it's
> > going to stay as is.
> 
> If the polling interval isn't accurate, the regression isn't so much about the
> timer being inaccurate, but more so in the fact that it'll take that much
> longer to react and increase the device frequency. Frame rendering time is
> 16ms. If you react after 20ms instead of 10ms, that's way past a frame
> rendering time. System memory frequency matters a lot for frame rendering too.

If you need precise timers use hrtimers and not a tick based mechanism,
it's that simple.

Thanks,

	tglx