linux-kernel - Re: [.32-rc3] scheduler: iwlagn consistently high in "waiting for CPU"

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Date:	Thu, 8 Oct 2009 22:34:26 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Mike Galbraith <efault@....de>
Cc:	Frans Pop <elendil@...net.nl>,
	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-wireless@...r.kernel.org
Subject: Re: [.32-rc3] scheduler: iwlagn consistently high in "waiting for
 CPU"

On Thu, Oct 08, 2009 at 08:23:37PM +0200, Mike Galbraith wrote:
> On Thu, 2009-10-08 at 16:55 +0200, Frans Pop wrote:
> > On Thursday 08 October 2009, Arjan van de Ven wrote:
> > > From: Arjan van de Ven <arjan@...ux.intel.com>
> > > Date: Thu, 24 Sep 2009 13:24:16 +0200
> > > Subject: [PATCH] x86, timers: check for pending timers after (device)
> > > interrupts
> > >
> > > Now that range timers and deferred timers are common, I found a
> > > problem with these using the "perf timechart" tool.
> > >
> > > It turns out that on x86, these two 'opportunistic' timers only
> > > get checked when another "real" timer happens.
> > > These opportunistic timers have the objective to save power by
> > > hitchhiking on other wakeups, as to avoid CPU wakeups by themselves
> > > as much as possible.
> > 
> > This patch makes quite a difference for me. iwlagn and phy0 now 
> > consistently show at ~10 ms or lower.
> > 
> > I do still get occasional high latencies, but those are for things like
> > "[rpc_wait_bit_killable]" or "Writing a page to disk", where I guess you'd 
> > expect them. Those high latencies are mostly only listed for "Global" and 
> > don't translate to individual processes.
> 
> I still see very high latencies coming out of idle (last noted was >
> 300ms, NO_HZ) with this patch, and wonder if the hunk below makes any
> difference whatsoever for you.  Here, it definitely does. (shouldn't)

I'm also seeing these strange, very high latencies here. Your patch
didn't help unfortunately.

This is from an otherwise idle NO_NZ system:

# ./perf sched latency

 -----------------------------------------------------------------------------------------
  Task                  |   Runtime ms  | Switches | Average delay ms | Maximum delay ms |
 -----------------------------------------------------------------------------------------
  ksoftirqd/0:4         |      2.216 ms |      170 | avg:   24.235 ms | max:  808.356 ms |
  ksoftirqd/1:6         |      2.611 ms |      205 | avg:    4.334 ms | max:  165.553 ms |
  migration/2:7         |      0.000 ms |        1 | avg:    3.060 ms | max:    3.060 ms |

With latencytop the ksoftirqd latency is over 1 sec frequently. (Could be
ondemand CPUfreq governor related?)

-- 
Markus
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