Re: [BUG nohz]: wrong user and system time accounting

[Wanpeng Li <kernellwp@gmail.com>]

2017-03-28 2:38 GMT+08:00 Luiz Capitulino <lcapitulino@redhat.com>:
> On Mon, 27 Mar 2017 09:56:47 +0800
> Wanpeng Li <kernellwp@gmail.com> wrote:
>
>> Actually after I bisect, the first bad commit is ff9a9b4c4334 ("sched,
>> time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity"). The bug
>> can be reproduced readily if CONFIG_CONTEXT_TRACKING_FORCE is true,
>> then just stress all the online cpus or just one cpu and leave others
>> idle(so it stresses the global timekeeping one), top show 100%
>> sys-time. And another way to reproduce it is by nohz_full, and gives
>> the stress to the house keeping cpu, the top show 100% sys-time of the
>> house keeping cpu, and also the other cpus who have at least two tasks
>> running on and in full_nohz mode.
>
> We're not short on reproducers, I have a new one too:
>
>  http://people.redhat.com/~lcapitul/real-time/acct-bug.c
>
> This is a single threaded task that reproduces the issue. If you
> run it as instructed, you'll get:
>
>  - nohz_full CPU: 95% system time 5% idle time
>  - non-nohz_full CPU: 95% user time 5% idle time (expected behavior)
>
> This reproduces the issue, but not for the reasons I expected. I was
> trying to mimic what I was seeing on my trace when tracing the two
> task problem. Which is: a task stays 995us in user-space and then
> enters the kernel. Time won't be accounted for user-space because
> we're not 1 jiffies yet, but if the task stays in the kernel for more
> than 5us, then time will be accounted for system time when going
> back to user-space.
>
> However, what really seems to be happening is: acct-bug is causing
> the tick to be re-activated (why? it shouldn't) and that causes the
> issue to appear. This is consistent with my other observations: I
> can only reproduce the issue if the nohz_full CPU re-activates the tick.

I see there are other kthreads like migration, kworker,
torture_shuffle etc on the isolated CPU.

Regards,
Wanpeng Li

>
>> Let's consider the cpu which has responsibility for the global
>> timekeeping, as the tracing posted above, the vtime_account_user() is
>> called before tick_sched_timer() which will update jiffies,
>
> But the vtime_account_user() call and the jiffies update happen
> on different CPUs, no? So the ordering shouldn't matter.
>
>> so jiffies
>> is stale in vtime_account_user() and the run time in userspace is
>> skipped, the vtime_user_enter() is called after jiffies update, so
>> both the time in userspace and in  kernel are accumulated to sys time.
>>
>> If the housekeeping cpu is idle when CONFIG_NO_HZ_FULL, everything is
>> fine. However, if you give stress to the housekeeping cpu, top will
>> show 100% sys-time of both the housekeeping cpu and the other cpus who
>> have at least two tasks running on and in full_nohz mode.
>
> The housekeeping CPUs are idle with my reproducers.
>
>> I think it
>> is because the stress delays the timer interrupt handling in some
>> degree, then the jiffies is not updated timely before other cpus
>> access it in vtime_account_user().
>>
>> I think we can keep syscalls/exceptions context tracking still in
>> jiffies based sampling and utilize local_clock() in vtime_delta()
>> again for irqs which avoids jiffies stale influence. I can make a
>> patch if the idea is acceptable or there is any better proposal. :)
>>
>> Regards,
>> Wanpeng Li
>>
>