linux-kernel - [PATCH 2/2] softirq: Don't skip softirq execution when softirq thread is parking

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Date:   Mon, 28 Jan 2019 15:46:25 -0800
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: [PATCH 2/2] softirq: Don't skip softirq execution when softirq thread is parking

When a CPU is unplugged the kernel threads of this CPU are parked
(see smpboot_park_threads()). kthread_park() is used to mark each
thread as parked and wake it up, so it can complete the process of
parking itselfs (see smpboot_thread_fn()).

If local softirqs are pending on interrupt exit invoke_softirq() is
called to process the softirqs, however it skips processing when the
softirq kernel thread of the local CPU is scheduled to run. The
softirq kthread is one of the threads that is parked when a CPU is
unplugged. Parking the kthread wakes it up, however only to complete
the parking process, not to process the pending softirqs. Hence
processing of softirqs at the end of an interrupt is skipped, but
not done elsewhere, which can result in warnings about pending
softirqs when a CPU is unplugged:

/sys/devices/system/cpu # echo 0 > cpu4/online
[ ... ] NOHZ: local_softirq_pending 02
[ ... ] NOHZ: local_softirq_pending 202
[ ... ] CPU4: shutdown
[ ... ] psci: CPU4 killed.

Don't skip processing of softirqs at the end of an interrupt when
the softirq thread of the CPU is parking.

Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
---
 kernel/softirq.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index d28813306b2c2..10277429ed84f 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -89,7 +89,8 @@ static bool ksoftirqd_running(unsigned long pending)

 	if (pending & SOFTIRQ_NOW_MASK)
 		return false;
-	return tsk && (tsk->state == TASK_RUNNING);
+	return tsk && (tsk->state == TASK_RUNNING) &&
+		!__kthread_should_park(tsk);
 }

 /*
-- 
2.20.1.495.gaa96b0ce6b-goog