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

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Message-ID: <1465412700.4639.10.camel@redhat.com>
Date:	Wed, 08 Jun 2016 15:05:00 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	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 Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v5 3/3] sched/cputime: Add steal time support to full
 dynticks CPU time accounting

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.

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