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From:   Thomas Gleixner <tglx@linutronix.de>
To:     paulmck@kernel.org, peterz@infradead.org
Cc:     Valentin Schneider <valentin.schneider@arm.com>,
        Vladimir Oltean <olteanv@gmail.com>,
        Kurt Kanzenbach <kurt.kanzenbach@linutronix.de>,
        Alison Wang <alison.wang@nxp.com>, catalin.marinas@arm.com,
        will@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>
Subject: Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ time accounting
In-Reply-To: <20200806132710.GL4295@paulmck-ThinkPad-P72>
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Date:   Thu, 06 Aug 2020 21:03:24 +0200
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Paul,

"Paul E. McKenney" <paulmck@kernel.org> writes:
> On Thu, Aug 06, 2020 at 01:45:45PM +0200, peterz@infradead.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