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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     Vladimir Oltean <olteanv@gmail.com>,
        Kurt Kanzenbach <kurt.kanzenbach@linutronix.de>,
        Alison Wang <alison.wang@nxp.com>, catalin.marinas@arm.com,
        will@kernel.org, paulmck@kernel.org, mw@semihalf.com,
        leoyang.li@nxp.com, vladimir.oltean@nxp.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Anna-Maria Gleixner <anna-maria@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ time accounting
In-Reply-To: <jhjh7tjivew.mognet@arm.com>
References: <20200729033934.22349-1-alison.wang@nxp.com> <877dumbtoi.fsf@kurt> <20200729094943.lsmhsqlnl7rlnl6f@skbuf> <87mu3ho48v.fsf@kurt> <20200730082228.r24zgdeiofvwxijm@skbuf> <873654m9zi.fsf@kurt> <87lfiwm2bj.fsf@nanos.tec.linutronix.de> <20200803114112.mrcuupz4ir5uqlp6@skbuf> <87d047n4oh.fsf@nanos.tec.linutronix.de> <jhjh7tjivew.mognet@arm.com>
Date:   Mon, 03 Aug 2020 21:22:53 +0200
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Valentin,

Valentin Schneider <valentin.schneider@arm.com> writes:
> On 03/08/20 16:13, Thomas Gleixner wrote:
>> Vladimir Oltean <olteanv@gmail.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