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: <20210315231117.GP2696@paulmck-ThinkPad-P72>
Date:   Mon, 15 Mar 2021 16:11:17 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...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 Fri, Mar 12, 2021 at 12:36:41PM +0100, Frederic Weisbecker wrote:
> 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>

Applied, thank you!

							Thanx, Paul