linux-kernel - Re: [PATCH v2] sched/cputime: steal_account_process

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Date:	Sun, 6 Mar 2016 11:58:57 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Chris Friesen <cbf123@...l.usask.ca>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	John Stultz <john.stultz@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v2] sched/cputime: steal_account_process_tick() should
 return jiffies

On Sat, 5 Mar 2016, Chris Friesen wrote:

> The callers of steal_account_process_tick() expect it to return
> whether a jiffy should be considered stolen or not.
> 
> Currently the return value of steal_account_process_tick() is in
> units of cputime, which vary between either jiffies or nsecs
> depending on CONFIG_VIRT_CPU_ACCOUNTING_GEN.
> 
> If cputime has nsecs granularity and there is a tiny amount of
> stolen time (a few nsecs, say) then we will consider the entire
> tick stolen and will not account the tick on user/system/idle,
> causing /proc/stats to show invalid data.
> 
> The fix is to change steal_account_process_tick() to accumulate
> the stolen time and only account it once it's worth a jiffy.
> 
> (Thanks to Frederic Weisbecker for suggestions to fix a bug in my
> first version of the patch.)
> 
> Signed-off-by: Chris Friesen <chris.friesen@...driver.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>