sched: account system time properly

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 <[email protected]>
Re-reported-by: Andrew Morton <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Acked-by: Martin Schwidefsky <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Paul Mackerras <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
LKML-Reference: <49F84BC1.7080602@cosmosbay.com>
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/kernel/sched.c b/kernel/sched.c
index b902e587a3a03e32a3e26f676e419f692ba79145..26efa475bdc143f6e4459067c18ce57e71608764 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4732,7 +4732,7 @@ void account_process_tick(struct task_struct *p, int user_tick)
 
        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,
                                    one_jiffy_scaled);
        else