Date: Sun, 6 Mar 2016 11:58:57 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Chris Friesen <cbf123@mail.usask.ca>
cc: Frederic Weisbecker <fweisbec@gmail.com>,
        John Stultz <john.stultz@linaro.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH v2] sched/cputime: steal_account_process_tick() should
 return jiffies
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References: <56DA1339.5030601@mail.usask.ca> <alpine.DEB.2.11.1603051121550.3686@nanos> <20160305131856.GA4441@lerouge> <56DBAF6D.5010503@mail.usask.ca> <56DBBDB8.40305@mail.usask.ca>
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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@windriver.com>

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