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

[Frederic Weisbecker]

On Wed, Mar 03, 2021 at 04:00:15PM -0800, paulmck@xxxxxxxxxx wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxx>
> Reported-by: Uladzislau Rezki <urezki@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> [ paulmck: Remove unneeded check per Sebastian Siewior feedback. ]
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>