linux-kernel - Re: [PATCH v2] sched/cputime: Resync steal time when guest & host lose sync

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Message-ID: <1471398894.32433.58.camel@redhat.com>
Date:	Tue, 16 Aug 2016 21:54:54 -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 <peterz@...radead.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Mike Galbraith <efault@....de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] sched/cputime: Resync steal time when guest & host
 lose sync

On Wed, 2016-08-17 at 09:16 +0800, Wanpeng Li wrote:
> 
> @@ -694,6 +699,12 @@ static cputime_t get_vtime_delta(struct
> task_struct *tsk)
>  	unsigned long now = READ_ONCE(jiffies);
>  	cputime_t delta, other;
>  
> +	/*
> +	 * The interval returned by account_other_time() is NOT
> +	 * rounded down to the nearest jiffy, while the base
> +	 * interval it is subtracted from is. So the max cputime
> +	 * limit is required to avoid underflow.
> +	 */
>  	delta = jiffies_to_cputime(now - tsk->vtime_snap);
>  	other = account_other_time(delta);
>  	WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE);

That comment makes sense in the context of the discussion
we have been having over the past few days, but could be
somewhat cryptic to someone looking at it 3 years from now.

How about something like the following?

	/*
	 * Unlike tick based timing, vtime based timing never has lost
	 * ticks, and no need for steal time accounting to make up for
	 * lost ticks. Vtime accounts a rounded version of actual
	 * elapsed time. Limit account_other_time to prevent rounding
	 * errors from causing elapsed vtime to go negative.
	 */		

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