linux-kernel - Re: 100% C0 with 2.6.25-rc

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Date:	Fri, 29 Feb 2008 10:24:32 -0800
From:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
To:	Len Brown <lenb@...nel.org>
Cc:	Jan Willies <jan@...lies.info>, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-acpi@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: 100% C0 with 2.6.25-rc

On Fri, Feb 22, 2008 at 09:03:55AM -0800, Pallipadi, Venkatesh wrote:
>  
> 
> >-----Original Message-----
> >From: Jan Willies [mailto:jan@...lies.info] 
> >Sent: Friday, February 22, 2008 7:56 AM
> >To: Rafael J. Wysocki
> >Cc: linux-acpi@...r.kernel.org; Ingo Molnar; LKML; Thomas 
> >Gleixner; Pallipadi, Venkatesh
> >Subject: Re: 100% C0 with 2.6.25-rc
> >
> >Rafael J. Wysocki wrote:
> >> On Thursday, 21 of February 2008, Jan Willies wrote:
> >>> Since 2.6.25-rc1 I have a lot of wakeups/s (≈134191,4) and 
> >spend 100% in C0.
> >>> It worked fine with 2.6.24 and commandline nolapic. Without 
> >nolapic I had 80k
> >>> wakeups/s after some time, but not right from the start like now.  
> >> 
> >> We have a regression from 2.6.24, apparently interrupts-related.
> >
> >After a lot of bisecting I've found the bad commit: 
> >
> >9b12e18cdc1553de62d931e73443c806347cd974 is first bad commit
> >commit 9b12e18cdc1553de62d931e73443c806347cd974
> >Author: venkatesh.pallipadi@...el.com <venkatesh.pallipadi@...el.com>
> >Date:   Thu Jan 31 17:35:05 2008 -0800
> >
> >    ACPI: cpuidle: Support C1 idle time accounting
> >    
> >    Show C1 idle time in /sysfs cpuidle interface. C1 idle time may not
> >    be entirely accurate in all cases. It includes the time spent
> >    in the interrupt handler after wakeup with "hlt" based C1. 
> >But, it will
> >    be accurate with "mwait" based C1.
> >
> >
> >Reverting the commit brings my laptop back to C2.
> >
> 
> Thanks for the bisect info. I will look at the bad side effects that patch
> may be having and I should have a patch for you to test later today....
> 

Jan replied later on this issue that he is not able to reproduce this problem
any more. Looking at the above, I could see a potential problem where
menu governor may get confused by the C-state residency time from poll
idle or C1 idle, where this timing info is not accurate. This inaccuracy is
due to interrupts being handled before we account for C-state exit.

Patch below fixes this and is needed regardless of the issue pointed out by
Jan Willies here. But, I am suspecting that the patch will also fix the
problem reported by Jan.

Do not mark TIME_VALID for CO poll state.
Mark C1 time as valid only with mwait (CSTATE_FFH) entry method.

This makes governors use the timing information only when it is correct and
eliminates any wrong policy decisions that may result from invalid timing
information.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>

Index: linux-2.6.git/drivers/acpi/processor_idle.c
===================================================================
--- linux-2.6.git.orig/drivers/acpi/processor_idle.c	2008-02-15 02:18:14.000000000 -0800
+++ linux-2.6.git/drivers/acpi/processor_idle.c	2008-02-26 07:02:30.000000000 -0800
@@ -1686,7 +1686,9 @@
 		switch (cx->type) {
 			case ACPI_STATE_C1:
 			state->flags |= CPUIDLE_FLAG_SHALLOW;
-			state->flags |= CPUIDLE_FLAG_TIME_VALID;
+			if (cx->entry_method == ACPI_CSTATE_FFH)
+				state->flags |= CPUIDLE_FLAG_TIME_VALID;
+
 			state->enter = acpi_idle_enter_c1;
 			dev->safe_state = state;
 			break;
Index: linux-2.6.git/drivers/cpuidle/cpuidle.c
===================================================================
--- linux-2.6.git.orig/drivers/cpuidle/cpuidle.c	2008-02-15 02:18:14.000000000 -0800
+++ linux-2.6.git/drivers/cpuidle/cpuidle.c	2008-02-22 03:59:01.000000000 -0800
@@ -224,7 +224,7 @@
 	state->exit_latency = 0;
 	state->target_residency = 0;
 	state->power_usage = -1;
-	state->flags = CPUIDLE_FLAG_POLL | CPUIDLE_FLAG_TIME_VALID;
+	state->flags = CPUIDLE_FLAG_POLL;
 	state->enter = poll_idle;
 }
 #else
--
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