linux-kernel - [PATCH RT 4/9] sched/core: Avoid __schedule() being called twice in a row

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Message-Id: <f627dfe1a81b884fbe95dc0591722331e343f0bb.1545347029.git.tom.zanussi@linux.intel.com>
Date:   Fri, 21 Dec 2018 09:21:16 -0600
From:   Tom Zanussi <zanussi@...nel.org>
To:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Cc:     rostedt@...dmis.org, tglx@...utronix.de, C.Emde@...dl.org,
        jkacur@...hat.com, bigeasy@...utronix.de,
        daniel.wagner@...mens.com, julia@...com
Subject: [PATCH RT 4/9] sched/core: Avoid __schedule() being called twice in a row

v3.18.129-rt111 rt-stable review patch.  If anyone has any objections,
please let me know.

------------------
From: Daniel Bristot de Oliveira <bristot@...hat.com>

[ Upstream commit 2bb94c48b2ffaabf8c15a51e5cc1b4c541988cab ]

If a worker invokes schedule() then we may have the call chain:
 schedule()
 -> sched_submit_work()
    -> wq_worker_sleeping()
       -> wake_up_worker()
	  -> wake_up_process().

The last wakeup may cause a schedule which is unnecessary because we are
already in schedule() and do it anyway.

Add a preempt_disable() + preempt_enable_no_resched() around
wq_worker_sleeping() so the context switch could be delayed until
__schedule().

Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Clark Williams <williams@...hat.com>
Cc: Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Cc: Romulo da Silva de Oliveira <romulo.deoliveira@...c.br>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
[bigeasy: rewrite changelog]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Tom Zanussi <tom.zanussi@...ux.intel.com>
---
 kernel/sched/core.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d5534f439b4a..500c319fcfc5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3114,10 +3114,15 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	/*
 	 * If a worker went to sleep, notify and ask workqueue whether
 	 * it wants to wake up a task to maintain concurrency.
+	 * As this function is called inside the schedule() context,
+	 * we disable preemption to avoid it calling schedule() again
+	 * in the possible wakeup of a kworker.
 	 */
-	if (tsk->flags & PF_WQ_WORKER)
+	if (tsk->flags & PF_WQ_WORKER) {
+		preempt_disable();
 		wq_worker_sleeping(tsk);
-
+		preempt_enable_no_resched();
+	}
 
 	if (tsk_is_pi_blocked(tsk))
 		return;
-- 
2.14.1