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

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Message-ID: <jhjh7tjivew.mognet@arm.com>
Date:   Mon, 03 Aug 2020 16:47:03 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
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

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.

I don't know if considering IRQ time in some way or another in
sched_rt_runtime_exceeded() really is a way out here.

> Thanks,
>
>         tglx