linux-kernel - Re: Rescheduling interrupts

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Date:	Tue, 13 May 2008 21:04:05 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"S.Çağlar Onur" <caglar@...dus.org.tr>,
	LKML <linux-kernel@...r.kernel.org>,
	"Arjan van de Ven" <arjan@...ux.intel.com>
Subject: Re: Rescheduling interrupts

On Tue, May 13, 2008 at 8:03 PM, Vegard Nossum <vegard.nossum@...il.com> wrote:
> Hi,
>
>
>  On Tue, Jan 22, 2008 at 6:05 PM, Ingo Molnar <mingo@...e.hu> wrote:
>  >
>  >  * S.Çağlar Onur <caglar@...dus.org.tr> wrote:
>  >
>  >
>  > > > My theory is that for whatever reason we get "repeat" IPIs: multiple
>  >  > > reschedule IPIs although the other CPU only initiated one.
>  >  >
>  >  > Ok, please see http://cekirdek.pardus.org.tr/~caglar/dmesg.3rd :)
>  >
>  >  hm, the IPI sending and receiving is nicely paired up:
>  >
>  >  [  625.795008] IPI (@smp_reschedule_interrupt) from task swapper:0 on CPU#1:
>  >  [  625.795223] IPI (@native_smp_send_reschedule) from task amarokapp:2882 on CPU#1:
>  >
>  >  amarokapp does wake up threads every 20 microseconds - that could
>  >  explain it. It's probably Xorg running on one core, amarokapp on the
>  >  other core. That's already 100 reschedules/sec.
>
>  Was this IPI thing ever resolved/fixed? I'm seeing the exact same
>  thing on my newest Fedora 8 kernel (2.6.24.5-85.fc8) now:
>
>   33.3% (334.0)      <kernel IPI> : Rescheduling interrupts
>
>  And I know for sure that this did not show up a couple of months back.
>
>  If the cause of this is known, please let me know about it. If it
>  isn't, I will start a bisect run now :-)

Ok -- just a confirmation. I have this on v2.6.26-rc1:

Running only init=/bin/sh (from busybox) and powertop shows:
kernel IPI : Rescheduling interrupts

completely idle box (except powertop): 1 wakeups/sec
one busy-loop[1]: 2 wakeups/sec
two busy-loops: 500 wakeups/sec

So it seems like a real bug to me.


Vegard

[1]: while true; do echo hi; done

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036