linux-kernel - Re: [PATCH 5/5] sched: Accumulate vtime on top of nsec clocksource

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Date:   Thu, 29 Jun 2017 19:27:27 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Frederic Weisbecker <fweisbec@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 5/5] sched: Accumulate vtime on top of nsec clocksource

On Thu, 2017-06-29 at 19:15 +0200, Frederic Weisbecker wrote:
> From: Wanpeng Li <kernellwp@...il.com>
> 
> Currently the cputime source used by vtime is jiffies. When we cross
> a context boundary and jiffies have changed since the last snapshot,
> the
> pending cputime is accounted to the switching out context.
> 
> This system works ok if the ticks are not aligned across CPUs. If
> they
> instead are aligned (ie: all fire at the same time) and the CPUs run
> in
> userspace, the jiffies change is only observed on tick exit and
> therefore
> the user cputime is accounted as system cputime. This is because the
> CPU that maintains timekeeping fires its tick at the same time as the
> others. It updates jiffies in the middle of the tick and the other
> CPUs
> see that update on IRQ exit:
> 
>     CPU 0 (timekeeper)                  CPU 1
>     -------------------              -------------
>                       jiffies = N
>     ...                              run in userspace for a jiffy
>     tick entry                       tick entry (sees jiffies = N)
>     set jiffies = N + 1
>     tick exit                        tick exit (sees jiffies = N + 1)
>                                                 account 1 jiffy as
> stime
> 
> Fix this with using a nanosec clock source instead of jiffies. The
> cputime is then accumulated and flushed everytime the pending delta
> reaches a jiffy in order to mitigate the accounting overhead.

Glad to hear this could be done without dramatically
increasing the accounting overhead!

> [fweisbec: changelog, rebase on struct vtime, field renames, add
> delta
> on cputime readers, keep idle vtime as-is (low overhead accounting),
> harmonize clock sources]
> 
> Reported-by: Luiz Capitulino <lcapitulino@...hat.com>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Not-Yet-Signed-off-by: Wanpeng Li <kernellwp@...il.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Wanpeng Li <kernellwp@...il.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Luiz Capitulino <lcapitulino@...hat.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>

Acked-by: Rik van Riel <riel@...hat.com>