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

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Message-ID: <34b6cca5355f9e709554892261ac4b0c@kernel.org>
Date:   Mon, 03 Aug 2020 14:24:39 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>,
        Vladimir Oltean <olteanv@...il.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Kurt Kanzenbach <kurt.kanzenbach@...utronix.de>,
        paulmck@...nel.org, Anna-Maria Gleixner <anna-maria@...utronix.de>,
        catalin.marinas@....com, Alison Wang <alison.wang@....com>,
        linux-kernel@...r.kernel.org, leoyang.li@....com,
        vladimir.oltean@....com, Thomas Gleixner <tglx@...utronix.de>,
        mw@...ihalf.com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ
 time accounting

On 2020-08-03 12:48, Valentin Schneider wrote:
> On 03/08/20 12:38, Vladimir Oltean wrote:
>> On Mon, Aug 03, 2020 at 10:51:32AM +0100, Robin Murphy wrote:
>>> 
>>> Having glanced across another thread that mentions IRQ accounting
>>> recently[1], I wonder if the underlying bug here might have something 
>>> do to
>>> with the stuff that Marc's trying to clean up.
>>> 
>>> Robin.
>>> 
>>> [1] 
>>> https://lore.kernel.org/linux-arm-kernel/20200624195811.435857-16-maz@kernel.org/
>> 
>> Thanks Robin. I've applied Marc's "[PATCH v2 00/17] arm/arm64: Turning
>> IPIs into normal interrupts" series and the LS1028A I'm debugging 
>> hangs
>> in absolutely the same way.
>> 
> 
> I'm not too surprised by that, wrt accounting this mostly changes where 
> the
> stores go to and barely shuffles when they happen (slightly earlier on 
> the
> IPI handling path).

Indeed. This series is just a "let's make things be the way they should 
be",
and isn't really fixing any bug. It actually may introduce a couple...

> FWIW I've had 'stress-ng --hrtimers 1' running on my Juno and eMAG for 
> ~15
> minutes and haven't had a splat yet.

I've started a couple of VMs with that workload too. 400K irq/s on an
8 vcpu guest, nothing to report so far.

But removing IRQ_TIME_ACCOUNTING from defconfig is a resounding "no 
way!".
It looks like we have a bug, and it should be squashed, not glanced 
over.

         M.
-- 
Jazz is not dead. It just smells funny...