linux-kernel - Re: [PATCH] sched/cputime: add steal time support to full dynticks CPU time accounting

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Message-ID: <CANRm+Cz3bsKdHFT6SKWaXyvzGCXP3t-zkhFFfVXgvn8phuK9bw@mail.gmail.com>
Date:	Wed, 18 May 2016 15:45:24 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Radim <rkrcmar@...hat.com>
Subject: Re: [PATCH] sched/cputime: add steal time support to full dynticks
 CPU time accounting

2016-05-18 15:10 GMT+08:00 Rik van Riel <riel@...hat.com>:
> On Tue, 2016-05-10 at 13:34 +0800, Wanpeng Li wrote:
>>
>> +++ b/kernel/sched/cputime.c
>>
>> @@ -691,8 +691,11 @@ static cputime_t get_vtime_delta(struct
>> task_struct *tsk)
>>
>>  static void __vtime_account_system(struct task_struct *tsk)
>>  {
>> +     unsigned long steal_time = steal_account_process_tick();
>>       cputime_t delta_cpu = get_vtime_delta(tsk);
>>
>> +     delta_cpu = steal_time ? (delta_cpu -
>> +             jiffies_to_cputime(steal_time)) : delta_cpu;
>>       account_system_time(tsk, irq_count(), delta_cpu,
>> cputime_to_scaled(delta_cpu));
>>  }
>>
>
> Sorry to have to go back on my previous email, but
> this is now a NAK
>
> The above code can end up passing a negative number
> to account_system_time(), which in turn can cause a
> divide by zero in scale_stime()
>
> The code needs to ensure that if all the time that
> passed was accounted as steal time (which could be
> more jiffies than expected, due to remaining partial
> jiffies in steal_account_process_tick), the function
> does not call account_system_time().

Yeah, I will fix it in next version, thank Rik very much for debugging
with me to figure out the root cause of divide zero.

Regards,
Wanpeng Li