Re: Regression (bisected): Lots of wakeups when using battery

From: Mike Galbraith
Date: Sat Jul 31 2010 - 00:01:47 EST

(corrects cc)

On Fri, 2010-07-30 at 22:45 +0100, Jaime Velasco Juan wrote:
> Hi, testing 2.6.35-rc5 I noticed that when my laptop is on battery,
> there are bursts of wakeups (they seem random, but usually happens once
> each 5 sec. or so; they cause some audible noise). If the laptop is plugged
> it behaves well. In single user mode, there are usually 2 wakeups/sec with
> the system idle, but on battery an with the bug, it goes to more than 100 w/s.
> Bisection points to:
> commit 3310d4d38fbc514e7b18bd3b1eea8effdd63b5aa
> Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Thu Jun 17 18:02:37 2010 +0200
> nohz: Fix nohz ratelimit
> Chris Wedgwood reports that 39c0cbe (sched: Rate-limit nohz) causes a
> serial console regression, unresponsiveness, and indeed it does. The
> reason is that the nohz code is skipped even when the tick was already
> stopped before the nohz_ratelimit(cpu) condition changed.
> Move the nohz_ratelimit() check to the other conditions which prevent
> long idle sleeps.
> Reverting said commit seems to fix the issue.

And if you revert that, using a serial console becomes.. unpleasant.

The whole ratelimit nohz thing has been dropped for now. It's a nice
little optimization for high frequency switchers on an underutilized
box, but has caused more irritation than it's worth.


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