linux-kernel - Re: [BISECTED] Lots of "rescheduling IPIs" in powertop

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Message-ID: <19f34abd0805140011ybee42c7mf78d1ae97b234460@mail.gmail.com>
Date:	Wed, 14 May 2008 09:11:55 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Andi Kleen" <ak@...e.de>, "Thomas Gleixner" <tglx@...utronix.de>,
	"Andreas Herrmann" <andreas.herrmann3@....com>,
	"S.Çağlar Onur" <caglar@...dus.org.tr>,
	Valdis.Kletnieks@...edu, "Matt Mackall" <mpm@...enic.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [BISECTED] Lots of "rescheduling IPIs" in powertop

Hi,

On Wed, May 14, 2008 at 8:56 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
>  * Vegard Nossum <vegard.nossum@...il.com> wrote:
>
>  > Hi,
>  >
>  > Recap: powertop shows between 200-400 wakeups/second with the
>  > description "<kernel IPI>: Rescheduling interrupts" when all
>  > processors have load (e.g. I need to run two busy-loops on my 2-CPU
>  > system for this to show up).
>
>  ok, could you try the fix below? It was a mistake to make mwait use
>  dependent on power considerations - on a desktop CPU it is unlikely to
>  use more power than a simple HLT - and the IPIs are extra scheduling
>  latency and extra power used.

This fixes it for the desktop machine at least. I guess I should try
on the laptop as well, but that will have to wait. Time for school :-)

Thanks.

Vegard

>  -------------->
>  Subject: x86: remove mwait C-state capability
>  From: Ingo Molnar <mingo@...e.hu>
>  Date: Wed May 14 08:47:40 CEST 2008
>
>
>  Signed-off-by: Ingo Molnar <mingo@...e.hu>
>  ---
>   arch/x86/kernel/process.c |   11 +----------
>   1 file changed, 1 insertion(+), 10 deletions(-)
>
>  Index: linux/arch/x86/kernel/process.c
>  ===================================================================
>  --- linux.orig/arch/x86/kernel/process.c
>  +++ linux/arch/x86/kernel/process.c
>  @@ -99,15 +99,6 @@ static void mwait_idle(void)
>                 local_irq_enable();
>   }
>
>  -
>  -static int __cpuinit mwait_usable(const struct cpuinfo_x86 *c)
>  -{
>  -       if (force_mwait)
>  -               return 1;
>  -       /* Any C1 states supported? */
>  -       return c->cpuid_level >= 5 && ((cpuid_edx(5) >> 4) & 0xf) > 0;
>  -}
>  -
>   /*
>   * On SMP it's slightly faster (but much more power-consuming!)
>   * to poll the ->work.need_resched flag instead of waiting for the
>  @@ -131,7 +122,7 @@ void __cpuinit select_idle_routine(const
>                         " performance may degrade.\n");
>         }
>   #endif
>  -       if (cpu_has(c, X86_FEATURE_MWAIT) && mwait_usable(c)) {
>  +       if (cpu_has(c, X86_FEATURE_MWAIT)) {
>                 /*
>                  * Skip, if setup has overridden idle.
>                  * One CPU supports mwait => All CPUs supports mwait
>



-- 
"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
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