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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: linux-kernel@vger.kernel.org
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	tglx@linutronix.de,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: [PATCH] sched/core: Schedule new worker even if PI-blocked
Date: Fri, 16 Aug 2019 18:06:26 +0200
Message-ID: <20190816160626.12742-1-bigeasy@linutronix.de> (raw)

If a task is PI-blocked (blocking on sleeping spinlock) then we don't want to
schedule a new kworker if we schedule out due to lock contention because !RT
does not do that as well. A spinning spinlock disables preemption and a worker
does not schedule out on lock contention (but spin).

On RT the RW-semaphore implementation uses an rtmutex so
tsk_is_pi_blocked() will return true if a task blocks on it. In this case we
will now start a new worker which may deadlock if one worker is waiting on
progress from another worker. Since a RW-semaphore starts a new worker on !RT,
we should do the same on RT.

XFS is able to trigger this deadlock.

Allow to schedule new worker if the current worker is PI-blocked.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/sched/core.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3945,7 +3945,7 @@ void __noreturn do_task_dead(void)
 
 static inline void sched_submit_work(struct task_struct *tsk)
 {
-	if (!tsk->state || tsk_is_pi_blocked(tsk))
+	if (!tsk->state)
 		return;
 
 	/*
@@ -3961,6 +3961,9 @@ static inline void sched_submit_work(str
 		preempt_enable_no_resched();
 	}
 
+	if (tsk_is_pi_blocked(tsk))
+		return;
+
 	/*
 	 * If we are going to sleep and we have plugged IO queued,
 	 * make sure to submit it to avoid deadlocks.

             reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-16 16:06 Sebastian Andrzej Siewior [this message]
2019-08-19  9:52 ` [tip:sched/urgent] " tip-bot for Sebastian Andrzej Siewior
2019-08-20 13:50 ` [PATCH] " Peter Zijlstra
2019-08-20 14:59   ` Sebastian Andrzej Siewior
2019-08-20 15:20     ` Peter Zijlstra
2019-08-20 15:54       ` Sebastian Andrzej Siewior
2019-08-20 16:02         ` Peter Zijlstra
2019-08-20 16:14           ` Sebastian Andrzej Siewior

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