linux-kernel - Re: [PATCH v4 3/3] sched/cputime: Add steal time support to full dynticks CPU time accounting

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Date:	Tue, 7 Jun 2016 19:23:46 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v4 3/3] sched/cputime: Add steal time support to full
 dynticks CPU time accounting

2016-06-07 18:47 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> On 07/06/2016 10:00, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>
>> This patch adds guest steal-time support to full dynticks CPU
>> time accounting. After the following commit:
>>
>> ff9a9b4c4334 ("sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity")
>>
>> ... time sampling became jiffy based, even if it's still listened
>> to ring boundaries, so steal_account_process_tick() is reused
>> to account how many 'ticks' are stolen-time, after the last accumulation.
>
> I still have no idea how to parse this.  What are "ring boundaries"?
> Rik, can you suggest a better commit message?

It is original from this slides.
http://ertl.jp/~shinpei/conf/ospert13/slides/FredericWeisbecker.pdf,
slide 28.

>
>> Suggested-and-Reviewed-by: Rik van Riel <riel@...hat.com>
>
> Please split Suggested-by and Reviewed-by.
>
>> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
>> index 75f98c5..9ff036b 100644
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -257,7 +257,7 @@ void account_idle_time(cputime_t cputime)
>>               cpustat[CPUTIME_IDLE] += (__force u64) cputime;
>>  }
>>
>> -static __always_inline bool steal_account_process_tick(void)
>> +static __always_inline unsigned long steal_account_process_tick(void)
>>  {
>>  #ifdef CONFIG_PARAVIRT
>>       if (static_key_false(&paravirt_steal_enabled)) {
>> @@ -279,7 +279,7 @@ static __always_inline bool steal_account_process_tick(void)
>>               return steal_jiffies;
>>       }
>>  #endif
>> -     return false;
>> +     return 0;
>>  }
>>
>>  /*
>> @@ -691,9 +691,13 @@ static cputime_t get_vtime_delta(struct task_struct *tsk)
>>
>>  static void __vtime_account_system(struct task_struct *tsk)
>>  {
>> -     cputime_t delta_cpu = get_vtime_delta(tsk);
>> +     cputime_t delta_time = get_vtime_delta(tsk);
>> +     cputime_t steal_time = jiffies_to_cputime(steal_account_process_tick());
>>
>> -     account_system_time(tsk, irq_count(), delta_cpu, cputime_to_scaled(delta_cpu));
>> +     if (steal_time < delta_time) {
>> +             delta_time -= steal_time;
>> +             account_system_time(tsk, irq_count(), delta_time, cputime_to_scaled(delta_time));
>> +     }
>>  }
>>
>>  void vtime_account_system(struct task_struct *tsk)
>> @@ -718,13 +722,18 @@ void vtime_gen_account_irq_exit(struct task_struct *tsk)
>>
>>  void vtime_account_user(struct task_struct *tsk)
>>  {
>> -     cputime_t delta_cpu;
>> +     cputime_t delta_time, steal_time;
>>
>>       write_seqcount_begin(&tsk->vtime_seqcount);
>>       tsk->vtime_snap_whence = VTIME_SYS;
>>       if (vtime_delta(tsk)) {
>> -             delta_cpu = get_vtime_delta(tsk);
>> -             account_user_time(tsk, delta_cpu, cputime_to_scaled(delta_cpu));
>> +             delta_time = get_vtime_delta(tsk);
>> +             steal_time = jiffies_to_cputime(steal_account_process_tick());
>> +
>> +             if (steal_time < delta_time) {
>> +                     delta_time -= steal_time;
>> +                     account_user_time(tsk, delta_time, cputime_to_scaled(delta_time));
>> +             }
>>       }
>>       write_seqcount_end(&tsk->vtime_seqcount);
>>  }
>>
>
> You're adding almost the same code to two callers of get_vtime_delta out
> of three.  I don't know the vtime accounting code very well, but why
> doesn't the same apply to account_idle_time?
>
> If it does, you should instead change get_vtime_delta to process steal
> time and subtract it from the result.
>
> Secondarily, when can it happen that steal_time > delta_time?
>
> Thanks,
>
> Paolo



-- 
Regards,
Wanpeng Li