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

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Date:   Mon, 03 Aug 2020 12:49:36 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kurt Kanzenbach <kurt.kanzenbach@...utronix.de>,
        Vladimir Oltean <olteanv@...il.com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ time accounting

Kurt,

Kurt Kanzenbach <kurt.kanzenbach@...utronix.de> writes:
> On Thu Jul 30 2020, Vladimir Oltean wrote:
> OK. I've reproduced it on a Marvell Armada SoC with v5.6 mainline. See
> splats below. Running with irq time accounting enabled, kills the
> machine immediately. However, I'm not getting the possible deadlock
> warnings in 8250 as you did. So that might be unrelated.
>
> Unfortunately I have no idea what to debug here.

lets look at the facts we have so far.

 1) When irq accounting is disabled, RT throttling kicks in as
    expected.

 2) With irq accounting the RT throttler does not kick in and the RCU
    stall/lockups happen.

Not much, but there is clearly interaction between irq time accounting
and scheduler accounting.

Can you please reduce /proc/sys/kernel/sched_rt_runtime_us in steps of
50000? At least in theory, reduction should bring the throttling back.

Thanks,

        tglx