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

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Date:   Wed, 12 Apr 2017 16:57:58 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <fweisbec@...il.com>
cc:     Wanpeng Li <kernellwp@...il.com>, Mike Galbraith <efault@....de>,
        Rik van Riel <riel@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Wed, 12 Apr 2017, Frederic Weisbecker wrote:
> On Tue, Apr 11, 2017 at 04:22:48PM +0200, Thomas Gleixner wrote:
> > It's not different from the current jiffies based stuff at all. Same
> > failure mode.
> 
> Yes you're right, I got confused again. So to fix this we could do our snapshots
> at a frequency lower than HZ but still high enough to avoid overhead.
> 
> Something like TICK_NSEC / 2 ?

If you are using TSC anyway then you can do proper accumulation for both
system and user and only account the data when the accumulation is more
than a jiffie.

Thanks,

	tglx