linux-kernel - Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ time accounting

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Date:   Mon, 03 Aug 2020 21:22:53 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        Kurt Kanzenbach <kurt.kanzenbach@...utronix.de>,
        Alison Wang <alison.wang@....com>, catalin.marinas@....com,
        will@...nel.org, paulmck@...nel.org, mw@...ihalf.com,
        leoyang.li@....com, vladimir.oltean@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ time accounting

Valentin,

Valentin Schneider <valentin.schneider@....com> writes:
> On 03/08/20 16:13, Thomas Gleixner wrote:
>> Vladimir Oltean <olteanv@...il.com> writes:
>>>>  1) When irq accounting is disabled, RT throttling kicks in as
>>>>     expected.
>>>> 
>>>>  2) With irq accounting the RT throttler does not kick in and the RCU
>>>>     stall/lockups happen.
>>> What is this telling us?
>>
>> It seems that the fine grained irq time accounting affects the runtime
>> accounting in some way which I haven't figured out yet.
>>
>
> With IRQ_TIME_ACCOUNTING, rq_clock_task() will always be incremented by a
> lesser-or-equal value than when not having the option; you start with the
> same delta_exec but slice some for the IRQ accounting, and leave the rest
> for the rq_clock_task() (+paravirt).
>
> IIUC this means that if you spend e.g. 10% of the time in IRQ and 90% of
> the time running the stress-ng RT tasks, despite having RT tasks hogging
> the entirety of the "available time" it is still only 90% runtime, which is
> below the 95% default and the throttling doesn't happen.

   totaltime = irqtime + tasktime

Ignoring irqtime and pretending that totaltime is what the scheduler
can control and deal with is naive at best.

Thanks,

        tglx