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SubjectRe: [.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@linux.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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