linux-kernel - Re: [stable] 2.6.23 regression: top displaying 9999% CPU usage

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Date:	Mon, 29 Oct 2007 21:04:25 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	Frans Pop <elendil@...net.nl>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Chuck Ebbert <cebbert@...hat.com>, Greg KH <greg@...ah.com>,
	stable@...nel.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [stable] 2.6.23 regression: top displaying 9999% CPU usage


* Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> We'll I initially thought of it and then I remembered that the 
> regression occurs only when the accounting itself is inaccurate. I am 
> tempted to ack the removal, but I would like to get input from others. 
> Meanwhile, I'll try and see if I can fix the problem

i've got a patch from Peter queued up. (see below) This should fix the 
main issue.

	Ingo

-------------------->
Subject: sched: keep utime/stime monotonic
From: Peter Zijlstra <a.p.zijlstra@...llo.nl>

keep utime/stime monotonic.

cpustats use utime/stime as a ratio against sum_exec_runtime, as a
consequence it can happen - when the ratio changes faster than time
accumulates - that either can be appear to go backwards.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 fs/proc/array.c       |    3 ++-
 include/linux/sched.h |    1 +
 kernel/fork.c         |    1 +
 3 files changed, 4 insertions(+), 1 deletion(-)

Index: linux/fs/proc/array.c
===================================================================
--- linux.orig/fs/proc/array.c
+++ linux/fs/proc/array.c
@@ -358,7 +358,8 @@ static cputime_t task_utime(struct task_
 	}
 	utime = (clock_t)temp;
 
-	return clock_t_to_cputime(utime);
+	p->prev_utime = max(p->prev_utime, clock_t_to_cputime(utime));
+	return p->prev_utime;
 }
 
 static cputime_t task_stime(struct task_struct *p)
Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -1009,6 +1009,7 @@ struct task_struct {
 	unsigned int rt_priority;
 	cputime_t utime, stime, utimescaled, stimescaled;
 	cputime_t gtime;
+	cputime_t prev_utime;
 	unsigned long nvcsw, nivcsw; /* context switch counts */
 	struct timespec start_time; 		/* monotonic time */
 	struct timespec real_start_time;	/* boot based time */
Index: linux/kernel/fork.c
===================================================================
--- linux.orig/kernel/fork.c
+++ linux/kernel/fork.c
@@ -1056,6 +1056,7 @@ static struct task_struct *copy_process(
 	p->gtime = cputime_zero;
 	p->utimescaled = cputime_zero;
 	p->stimescaled = cputime_zero;
+	p->prev_utime = cputime_zero;
 
 #ifdef CONFIG_TASK_XACCT
 	p->rchar = 0;		/* I/O counter: bytes read */
-
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