linux-kernel - Re: [PATCH] sched/cputime: Fix ksoftirqd cputime accounting regression

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Date:   Wed, 26 Apr 2017 11:44:04 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stanislaw Gruszka <sgruszka@...hat.com>
Subject: Re: [PATCH] sched/cputime: Fix ksoftirqd cputime accounting
 regression

On Tue, 2017-04-25 at 16:10 +0200, Frederic Weisbecker wrote:
> irq_time_read() returns the irqtime minus the ksoftirqd time. This
> is necessary because irq_time_read() is used to substract the IRQ
> time
> from the sum_exec_runtime of a task. If we were to include the
> softirq
> time of ksoftirqd, this task would substract its own CPU time
> everytime
> it updates ksoftirqd->sum_exec_runtime which would therefore never
> progress.
> 
> But this behaviour got broken by commit a499a5a14db:
> 	("sched/cputime: Increment kcpustat directly on irqtime
> account")
> which now includes ksoftirqd softirq time in the time returned by
> irq_time_read().

Ahhh, after a look at account_system_time, it becomes
clear why that was happening!

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