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Subject[PATCH 3/4] sched: account system time properly
From: Eric Dumazet <>

[upstream commit f5f293a4e3d0a0c52cec31de6762c95050156516]
[this is not actually necessary for applying BFS, but is included for
the sake of completeness since it is mirrored in BFS]

Andrew Gallatin reported that IRQ and SOFTIRQ times were
sometime not reported correctly on recent kernels, and even
bisected to commit 457533a7d3402d1d91fbc125c8bd1bd16dcd3cd4
([PATCH] fix scaled & unscaled cputime accounting) as the first
bad commit.

Further analysis pointed that commit
79741dd35713ff4f6fd0eafd59fa94e8a4ba922d ([PATCH] idle cputime
accounting) was the real cause of the problem.

account_process_tick() was not taking into account timer IRQ
interrupting the idle task servicing a hard or soft irq.

On mostly idle cpu, irqs were thus not accounted and top or
mpstat could tell user/admin that cpu was 100 % idle, 0.00 %
irq, 0.00 % softirq, while it was not.

[ Impact: fix occasionally incorrect CPU statistics in top/mpstat ]

Reported-by: Andrew Gallatin <>
Re-reported-by: Andrew Morton <>
Signed-off-by: Eric Dumazet <>
Acked-by: Martin Schwidefsky <>
Cc: Paul Mackerras <>
Cc: Benjamin Herrenschmidt <>
LKML-Reference: <>
Signed-off-by: Ingo Molnar <>

kernel/sched.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff -udrNp linux-2.6.27.orig/kernel/sched.c linux-2.6.27/kernel/sched.c
--- linux-2.6.27.orig/kernel/sched.c 2009-10-02 05:14:57.590579107 -0500
+++ linux-2.6.27/kernel/sched.c 2009-10-02 05:15:19.947578991 -0500
@@ -4187,7 +4187,7 @@ void account_process_tick(struct task_st

if (user_tick)
account_user_time(p, one_jiffy, one_jiffy_scaled);
- else if (p != rq->idle)
+ else if ((p != rq->idle) || (irq_count() != HARDIRQ_OFFSET))
account_system_time(p, HARDIRQ_OFFSET, one_jiffy,

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