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

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Date:	Wed, 14 May 2008 08:56:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Vegard Nossum <vegard.nossum@...il.com>
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


* 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.

	Ingo

-------------->
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
--
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