linux-kernel - Re: [PATCH tip/core/rcu 06/10] softirq: Don't try waking ksoftirqd before it has been spawned

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Message-ID: <20210312113641.GA3646@lothringen>
Date:   Fri, 12 Mar 2021 12:36:41 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 06/10] softirq: Don't try waking ksoftirqd
 before it has been spawned

On Wed, Mar 03, 2021 at 04:00:15PM -0800, paulmck@...nel.org wrote:
> From: "Paul E. McKenney" <paulmck@...nel.org>
> 
> If there is heavy softirq activity, the softirq system will attempt
> to awaken ksoftirqd and will stop the traditional back-of-interrupt
> softirq processing.  This is all well and good, but only if the
> ksoftirqd kthreads already exist, which is not the case during early
> boot, in which case the system hangs.
> 
> One reproducer is as follows:
> 
> tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 2 --configs "TREE03" --kconfig "CONFIG_DEBUG_LOCK_ALLOC=y CONFIG_PROVE_LOCKING=y CONFIG_NO_HZ_IDLE=y CONFIG_HZ_PERIODIC=n" --bootargs "threadirqs=1" --trust-make
> 
> This commit therefore adds a couple of existence checks for ksoftirqd
> and forces back-of-interrupt softirq processing when ksoftirqd does not
> yet exist.  With this change, the above test passes.
> 
> Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Reported-by: Uladzislau Rezki <urezki@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> [ paulmck: Remove unneeded check per Sebastian Siewior feedback. ]
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>