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

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Message-ID: <87k0ybha0z.fsf@nanos.tec.linutronix.de>
Date:   Thu, 06 Aug 2020 21:03:24 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     paulmck@...nel.org, 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, 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

Paul,

"Paul E. McKenney" <paulmck@...nel.org> writes:
> On Thu, Aug 06, 2020 at 01:45:45PM +0200, peterz@...radead.org wrote:
>> The safety thing is concerned with RT tasks. It doesn't pretend to help
>> with runnaway IRQs, never has, never will.
>
> Getting into the time machine back to the 1990s...
>
> DYNIX/ptx had a discretionary mechanism to deal with excessive interrupts.
> There was a function that long-running interrupt handlers were supposed
> to call periodically that would return false if the system felt that
> the CPU had done enough interrupts for the time being.  In that case,
> the interrupt handler was supposed to schedule itself for a later time,
> but leave the interrupt unacknowledged in order to prevent retriggering
> in the meantime.
>
> Of course, this mechanism would be rather less helpful in Linux.
>
> For one, Linux has way more device drivers and way more oddball devices.
> In contrast, the few devices that DYNIX/ptx supported were carefully
> selected, and the selection criteria included being able to put up
> with this sort of thing.  Also, the fact that there was but a handful
> of device drivers meant that changes like this could be more easily
> propagated through all drivers.

We could do that completely at the core interrupt handling level. 

> Also, Linux supports way more workloads.  In contrast, DYNIX/ptx could
> pick a small percentage of each CPU that would be permitted to be used
> by hardware interrupt handlers.  As in there are probably Linux workloads
> that run >90% of some poor CPU within hardware interrupt handlers.

Yet another tunable. /me runs