Re: [PATCH v5 3/3] sched/cputime: Add steal time support to full dynticks CPU time accounting

From: Wanpeng Li <kernellwp@gmail.com>
To: Rik van Riel <riel@redhat.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	kvm <kvm@vger.kernel.org>, "Wanpeng Li" <wanpeng.li@hotmail.com>,
	"Ingo Molnar" <mingo@kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Frederic Weisbecker" <fweisbec@gmail.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>
Subject: Re: [PATCH v5 3/3] sched/cputime: Add steal time support to full dynticks CPU time accounting
Date: Thu, 9 Jun 2016 07:57:58 +0800	[thread overview]
Message-ID: <CANRm+CzRAxp2G40ZdzdjUNUFMfOKo5BVxZ-xJzZ9L5oyy1ct2g@mail.gmail.com> (raw)
In-Reply-To: <1465412700.4639.10.camel@redhat.com>

2016-06-09 3:05 GMT+08:00 Rik van Riel <riel@redhat.com>:
> On Wed, 2016-06-08 at 11:05 +0800, Wanpeng Li wrote:
>>
>> @@ -681,12 +681,17 @@ static cputime_t vtime_delta(struct task_struct
>> *tsk)
>>  static cputime_t get_vtime_delta(struct task_struct *tsk)
>>  {
>>       unsigned long now = READ_ONCE(jiffies);
>> -     unsigned long delta = now - tsk->vtime_snap;
>> +     cputime_t delta_time, steal_time;
>>
>> +     steal_time =
>> jiffies_to_cputime(steal_account_process_tick());
>> +     delta_time = jiffies_to_cputime(now - tsk->vtime_snap);
>>       WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE);
>>       tsk->vtime_snap = now;
>>
>> -     return jiffies_to_cputime(delta);
>> +     if (steal_time < delta_time)
>> +             delta_time -= steal_time;
>> +
>> +     return delta_time;
>>  }
>
> This isn't right.
>
> If steal_time is equal to or larger than delta_time,
> get_vtime_delta needs to return 0, not delta_time.
>
> Otherwise the same time will be counted twice.

Paolo also pointed out this yesterday, so his proposal looks good to you, right?

Regards,
Wanpeng Li