Re: [PATCH] sched: fix cpu utilization account error

[Greg KH]

On Mon, Jul 01, 2013 at 02:45:04PM +0800, Xie XiuQi wrote:
> We setting clock_skip_update = 1 based on the assumption that the
> next call to update_rq_clock() will come nearly immediately
> after being set. However, it is not always true especially on
> non-preempt mode. In this case we may miss some clock update, which
> would cause an error curr->sum_exec_runtime account.
> 
> The test result show that test_kthread's exec_runtime has been
> added to watchdog.
> 
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+   P COMMAND
>    28 root      RT   0     0    0    0 S  100  0.0   0:05.39  5 watchdog/5
>     7 root      RT   0     0    0    0 S   95  0.0   0:05.83  0 watchdog/0
>    12 root      RT   0     0    0    0 S   94  0.0   0:05.79  1 watchdog/1
>    16 root      RT   0     0    0    0 S   92  0.0   0:05.74  2 watchdog/2
>    20 root      RT   0     0    0    0 S   91  0.0   0:05.71  3 watchdog/3
>    24 root      RT   0     0    0    0 S   82  0.0   0:05.42  4 watchdog/4
>    32 root      RT   0     0    0    0 S   79  0.0   0:05.35  6 watchdog/6
>  5200 root      20   0     0    0    0 R   21  0.0   0:08.88  6 test_kthread/6
>  5194 root      20   0     0    0    0 R   20  0.0   0:08.41  0 test_kthread/0
>  5195 root      20   0     0    0    0 R   20  0.0   0:08.44  1 test_kthread/1
>  5196 root      20   0     0    0    0 R   20  0.0   0:08.49  2 test_kthread/2
>  5197 root      20   0     0    0    0 R   20  0.0   0:08.53  3 test_kthread/3
>  5198 root      20   0     0    0    0 R   19  0.0   0:08.81  4 test_kthread/4
>  5199 root      20   0     0    0    0 R    2  0.0   0:08.66  5 test_kthread/5
> 
> "test_kthread/i" is a kernel thread which has a infinity loop and it calls
> schedule() every 1s. It's main process as below:
> 
> static int main_loop (void *unused)
> {
> 	unsigned long flags;
> 	unsigned long last = jiffies;
> 	int i;
> 
> 	while (!kthread_should_stop()) {
> 		/* call schedule every 1 sec */
> 		if (HZ <= jiffies - last) {
> 			last = jiffies;
> 			schedule();
> 		}
> 
> 		/* do some thing */
> 		for (i = 0; i < 1000; i++)
> 		;
> 
> 		if (kthread_should_stop())
> 			break;
> 	}
> }
> 
> In this patch, we do not skip clock update when current task is kernel
> thread in non-preempt mode.
> 
> Reported-by: Zhang Hang <bob.zhanghang@xxxxxxxxxx>
> Signed-off-by: Xie XiuQi <xiexiuqi@xxxxxxxxxx>
> ---
>  kernel/sched/core.c |   11 +++++++++++
>  1 files changed, 11 insertions(+), 0 deletions(-)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read Documentation/stable_kernel_rules.txt
for how to do this properly.

</formletter>
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