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

From: Thomas Gleixner <tglx@linutronix.de>
To: peterz@infradead.org
Cc: mw@semihalf.com, paulmck@kernel.org,
	Anna-Maria Gleixner <anna-maria@linutronix.de>,
	catalin.marinas@arm.com, Alison Wang <alison.wang@nxp.com>,
	linux-kernel@vger.kernel.org, leoyang.li@nxp.com,
	vladimir.oltean@nxp.com,
	Kurt Kanzenbach <kurt.kanzenbach@linutronix.de>,
	Vladimir Oltean <olteanv@gmail.com>,
	will@kernel.org, Valentin Schneider <valentin.schneider@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ time accounting
Date: Thu, 06 Aug 2020 11:34:42 +0200	[thread overview]
Message-ID: <877duci0ct.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200805134002.GQ2674@hirez.programming.kicks-ass.net>

peterz@infradead.org writes:
> On Mon, Aug 03, 2020 at 09:22:53PM +0200, Thomas Gleixner wrote:
>
>>    totaltime = irqtime + tasktime
>> 
>> Ignoring irqtime and pretending that totaltime is what the scheduler
>> can control and deal with is naive at best.
>
> Well no, that's what we call system overhead and is assumed to be
> included in the 'error margin'.
>
> The way things are set up is that we say that, by default, RT tasks can
> consume 95% of cputime and the remaining 5% is sufficient to keep the
> system alive.
>
> Those 5% include all system overhead, IRQs, RCU, !RT workqueues etc..
>
> Obviously IRQ_TIME accounting changes the balance a bit, but that's what
> it is. We can't really do anything better.
>
> Apparently this SoC has significant IRQ time for some reason. Also,
> relying on RT throttling for 'correct' behaviour is also wrong. What
> needs to be done is find who is using all this RT time and why, that
> isn't right.

It's a test case and we know already who is using the time. But that's
not the point.

A runaway RT task resulting in a RCU stall or whatever lockup of the
system is definitely not the right answer.

The throttler, as much as it's a horrible hack, is there to prevent this
and to give the admin a chance to pinpoint and kill that thing instead
of having to press the reset button and scratching head what might have
caused this.

Thanks,

        tglx

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