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

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Message-ID: <87mu37ha99.fsf@nanos.tec.linutronix.de>
Date:   Thu, 06 Aug 2020 20:58:26 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     peterz@...radead.org
Cc:     Valentin Schneider <valentin.schneider@....com>,
        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>
Subject: Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ time accounting

Peter,

peterz@...radead.org writes:
> On Thu, Aug 06, 2020 at 11:41:06AM +0200, Thomas Gleixner wrote:
>> And that has nothing to do
>> with a silly test case. Sporadic runaways due to a bug in a once per
>> week code path simply can happen and having the safety net working
>> depending on a config option selected or not is just wrong.
>
> The safety thing is concerned with RT tasks. It doesn't pretend to help
> with runnaway IRQs, never has, never will.

Of course not. But without irq accounting the runtime is accounted on
the runaway task which causes it to throttle.

> The further extreme is an interrupt storm, those have always taken a
> machine down.

If every interrupt is actually handled, then yes.

> Accounting unrelated IRQ time to RT tasks is equally wrong, the task
> execution is unrelated to the IRQs. The config option at least offers
> insight into where time goes -- and it's a config option because doing
> time accounting on interrupts adds overhead :/

Right, but it's not totally out of the world either to make the
throttler do:

          if (rt_runtime + irq_time > threshold)
          	try_to_keep_the_box_alive()

> This really is a no-win all round.

That's not the question here :)

> The only 'sensible' option here is threaded IRQs, where the IRQ line
> gets disabled until the handler thread has ran, that also helps with IRQ
> storms.

I'm not against enforcing threaded IRQs. :)

Thanks,

        tglx