linux-kernel - Re: [Bug #10710] [BISECTED] Lots of "rescheduling IPIs" in powertop

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Date:	Sun, 18 May 2008 19:59:41 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Vegard Nossum <vegard.nossum@...il.com>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Bug #10710] [BISECTED] Lots of "rescheduling IPIs" in powertop

Vegard Nossum wrote:
> On Sun, May 18, 2008 at 1:13 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>> This message has been generated automatically as a part of a report
>> of recent regressions.
>>
>> The following bug entry is on the current list of known regressions
>> from 2.6.25.  Please verify if it still should be listed.
>>
>>
>> Bug-Entry       : http://bugzilla.kernel.org/show_bug.cgi?id=10710
>> Subject         : [BISECTED] Lots of "rescheduling IPIs" in powertop
>> Submitter       : Vegard Nossum <vegard.nossum@...il.com>
>> Date            : 2008-05-13 20:42 (6 days old)
>> References      : http://marc.info/?l=linux-kernel&amp;m=121071176205864&amp;w=4
>> Handled-By      : Andi Kleen <andi@...stfloor.org>
>>                  Arjan van de Ven <arjan@...radead.org>
>>                  Ingo Molnar <mingo@...e.hu>
>> Patch           : http://marc.info/?l=linux-kernel&amp;m=121074826823352&amp;w=4
>>
> 
> Ingo Molnar provided the patch that solved the problem (the referenced
> link) but he wanted to wait for Andreas Herrmann to provide an
> additional patch that would solve some implication on AMD CPUs. So
> this is in theory fixed, but the fix has not yet been applied anywhere
> as far as I know.

There's a CPUID way to distingush the cases I found out now (with help
from Venki, thanks) so it would be possible to solve it properly, but
once even a bad Ingo patch is in it's nearly impossible to replace it
with something better so I bail out at this point.

[In case someone is interested it's CPUID 5 ECX bit 0 which enumerates
if the MWAIT enumeration is there. So the correct mwait_usable() that
would have avoided your problem would be something like (untested):

return c->cpuid_level >= 5 &&
           ((cpuid_ecx(5) & 1) == 0 || (cpuid_edx(5) >> 4) & 0xf) > 0);
]

-Andi
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