linux-kernel - Re: [BUG nohz]: wrong user and system time accounting

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Date:   Thu, 30 Mar 2017 14:40:28 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Rik van Riel <riel@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Thu, Mar 30, 2017 at 09:58:44AM +0800, Wanpeng Li wrote:
> 2017-03-30 4:08 GMT+08:00 Rik van Riel <riel@...hat.com>:
> >
> > In other words, the tick on cpu0 is aligned
> > with the tick on the nohz_full cpus, and
> > jiffies is advanced while the nohz_full cpus
> > with an active tick happen to be in kernel
> > mode?
> >
> > Frederic, can you think of any reason why
> > the tick on nohz_full CPUs would end up aligned
> > with the tick on cpu0, instead of running at some
> > random offset?
> >
> > A random offset, or better yet a somewhat randomized
> > tick length to make sure that simultaneous ticks are
> > fairly rare and the vtime sampling does not end up
> > "in phase" with the jiffies incrementing, could make
> > the accounting work right again.
> >
> > Of course, that assumes the above hypothesis is correct :)
> 
> There is such a feature skew_tick currently, refer to commit
> 5307c9556bc (tick: add tick skew boot option), w/ skew_tick=1 boot
> parameter, the bug disappear, however, the commit also mentioned that
> it will hurt power consumption.

Oh, I completely missed that!

> I will try Frederic's proposal which
> is similar to my original idea "how bad would it be to revert to
> sched_clock() instead of jiffies in vtime_delta()? We could use
> nanosecond granularity to check deltas but only perform an actual
> cputime update when that delta >= TICK_NSEC."

Thanks! I hope sched_clock() won't introduce too much overhead.
Otherwise we may want to pick up the skew_tick solution.