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

[Xie XiuQi <xiexiuqi@huawei.com>]

On 2013/7/1 15:36, Mike Galbraith wrote:
> On Mon, 2013-07-01 at 14:45 +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:
> 
> It'd be a shame to lose the cycle savings (we could use more) due to
> such horrible behavior.  Where are you seeing this in real life?
> 

Thank you for your comments, Mike.

This issue was reported by a driver related pcie in which a kthread send
huge amounts of data. In non-preempt mode, it would take a cpu for a long
time. But, in preempt mode, I haven't found this issue yet.

Here is the kthread main logic. Although it's not a good idea, but it does
exist:
while (!kthread_should_stop()) {
	/* call schedule every 1 sec */
	if (HZ <= jiffies - last) {
		last = jiffies;
		schedule();
	}

	/* get data and sent it */
	get_msg();
	send_msg();

	if (kthread_should_stop())
		break;
}

> That said, accounting funnies induced by skipped update are possible,
> which could trump the cycle savings I suppose, so maybe savings (sniff)
> should just go away?

Indeed, removing the skip_clock_update could resolve the issue, but I found
there is no this issue in preempt mode. However, if remove skip_clock_update
we'll get more precise time account.

So, what's your opinion, Mike.