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

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Message-ID: <CANRm+CwWj=XOaJeiauHPkzDbRgsa81AWxNNAfzTxOK_Q15M+xQ@mail.gmail.com>
Date:   Tue, 11 Apr 2017 19:03:17 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Frederic Weisbecker <fweisbec@...il.com>
Cc:     Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG nohz]: wrong user and system time accounting

2017-03-30 21:38 GMT+08:00 Frederic Weisbecker <fweisbec@...il.com>:
> On Thu, Mar 30, 2017 at 02:47:11PM +0800, Wanpeng Li wrote:

[...]

>
>>
>> -------------------------------------->8-----------------------------------------------------
>>
>> use nanosecond granularity to check deltas but only perform an actual
>> cputime update when that delta >= TICK_NSEC.
>>
>> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
>> index f3778e2b..f1ee393 100644
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -676,18 +676,21 @@ void thread_group_cputime_adjusted(struct
>> task_struct *p, u64 *ut, u64 *st)
>>  #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
>>  static u64 vtime_delta(struct task_struct *tsk)
>>  {
>> -    unsigned long now = READ_ONCE(jiffies);
>> +    u64 now = local_clock();
>
> I fear we need a global clock, because the reader (task_cputime()) needs
> to compute the delta and therefore use the same clock from any CPU.
>
> Or we can use the local_clock() but the reader must access the same.
>
> So there would be vtime_delta_writer() which uses local_clock and stores
> the current CPU to tsk->vtime_cpu (under the vtime_seqcount). And then
> vtime_delta_reader() which calls sched_clock_cpu(tsk->vtime_cpu) which
> is protected by vtime_seqcount as well.
>
> Although those sched_clock_cpu() things seem to only matter when the
> sched_clock() is unstable. And that stability is a condition for nohz_full
> to work anyway. So probably sched_clock() alone would be enough.

I observed ~60% user time and ~40% sys time when replace local_clock()
above by sched_clock()(two cpu hogs on the cpu in nohz_full mode). In
addition, Luiz's testcast ./acct-bug 1 995 will show 100% idle time.
If keep local_clock() in vtime_delta(), cpu hogs testcase will
success. However, Luiz's testcase still show 100% idle time.

Regards,
Wanpeng Li