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SubjectRe: [PATCH v2] sched/cputime: Resync steal time when guest & host lose sync
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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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