From: tip-bot for Thomas Gleixner <tipbot@zytor.com>
To: linux-tip-commits@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, hpa@zytor.com,
mingo@kernel.org, peterz@infradead.org, bigeasy@linutronix.de,
tglx@linutronix.de
Subject: [tip:sched/core] sched: Queue RT tasks to head when prio drops
Date: Fri, 21 Feb 2014 13:32:13 -0800 [thread overview]
Message-ID: <tip-410dcf7b5670c224f5bb3179b62642b7182e3486@git.kernel.org> (raw)
In-Reply-To: <1391803122-4425-6-git-send-email-bigeasy@linutronix.de>
Commit-ID: 410dcf7b5670c224f5bb3179b62642b7182e3486
Gitweb: http://git.kernel.org/tip/410dcf7b5670c224f5bb3179b62642b7182e3486
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Fri, 7 Feb 2014 20:58:41 +0100
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Fri, 21 Feb 2014 21:43:19 +0100
sched: Queue RT tasks to head when prio drops
The following scenario does not work correctly:
Runqueue of CPUx contains two runnable and pinned tasks:
T1: SCHED_FIFO, prio 80
T2: SCHED_FIFO, prio 80
T1 is on the cpu and executes the following syscalls (classic priority
ceiling scenario):
sys_sched_setscheduler(pid(T1), SCHED_FIFO, .prio = 90);
...
sys_sched_setscheduler(pid(T1), SCHED_FIFO, .prio = 80);
...
Now T1 gets preempted by T3 (SCHED_FIFO, prio 95). After T3 goes back
to sleep the scheduler picks T2. Surprise!
The same happens w/o actual preemption when T1 is forced into the
scheduler due to a sporadic NEED_RESCHED event. The scheduler invokes
pick_next_task() which returns T2. So T1 gets preempted and scheduled
out.
This happens because sched_setscheduler() dequeues T1 from the prio 90
list and then enqueues it on the tail of the prio 80 list behind T2.
This violates the POSIX spec and surprises user space which relies on
the guarantee that SCHED_FIFO tasks are not scheduled out unless they
give the CPU up voluntarily or are preempted by a higher priority
task. In the latter case the preempted task must get back on the CPU
after the preempting task schedules out again.
We fixed a similar issue already in commit 60db48c (sched: Queue a
deboosted task to the head of the RT prio queue). The same treatment
is necessary for sched_setscheduler(). So enqueue to head of the prio
bucket list if the priority of the task is lowered.
It might be possible that existing user space relies on the current
behaviour, but it can be considered highly unlikely due to the corner
case nature of the application scenario.
Cc: stable@vger.kernel.org
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1391803122-4425-6-git-send-email-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
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 7527e68..a41d239 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3450,8 +3450,13 @@ change:
if (running)
p->sched_class->set_curr_task(rq);
- if (on_rq)
- enqueue_task(rq, p, 0);
+ if (on_rq) {
+ /*
+ * We enqueue to tail when the priority of a task is
+ * increased (user space view).
+ */
+ enqueue_task(rq, p, oldprio <= p->prio ? ENQUEUE_HEAD : 0);
+ }
check_class_changed(rq, p, prev_class, oldprio);
task_rq_unlock(rq, p, &flags);
next prev parent reply other threads:[~2014-02-21 21:32 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-07 19:58 A pile of sched patches Sebastian Andrzej Siewior
2014-02-07 19:58 ` [PATCH 1/6] sched: Init idle->on_rq in init_idle() Sebastian Andrzej Siewior
2014-02-07 21:09 ` Peter Zijlstra
2014-02-11 9:17 ` [PATCH 1/6 v2] " Sebastian Andrzej Siewior
2014-02-11 9:21 ` Peter Zijlstra
2014-02-11 15:34 ` Thomas Gleixner
2014-02-11 15:51 ` Peter Zijlstra
2014-02-21 21:31 ` [tip:sched/core] " tip-bot for Thomas Gleixner
2014-02-22 18:01 ` tip-bot for Thomas Gleixner
2014-02-07 19:58 ` [PATCH 2/6] sched: Check for idle task in might_sleep() Sebastian Andrzej Siewior
2014-02-21 21:31 ` [tip:sched/core] " tip-bot for Thomas Gleixner
2014-02-22 18:02 ` tip-bot for Thomas Gleixner
2014-02-07 19:58 ` [PATCH 3/6] sched: Better debug output for might sleep Sebastian Andrzej Siewior
2014-02-21 21:31 ` [tip:sched/core] " tip-bot for Thomas Gleixner
2014-02-22 18:02 ` [tip:sched/core] sched: Add better debug output for might_sleep() tip-bot for Thomas Gleixner
2014-02-07 19:58 ` [PATCH 4/6] sched: Adjust sched_reset_on_fork when nothing else changes Sebastian Andrzej Siewior
2014-02-21 21:32 ` [tip:sched/core] " tip-bot for Thomas Gleixner
2014-02-22 18:02 ` [tip:sched/core] sched: Adjust p-> " tip-bot for Thomas Gleixner
2014-02-07 19:58 ` [PATCH 5/6] sched: Queue RT tasks to head when prio drops Sebastian Andrzej Siewior
2014-02-21 21:32 ` tip-bot for Thomas Gleixner [this message]
2014-02-22 18:02 ` [tip:sched/core] " tip-bot for Thomas Gleixner
2014-02-07 19:58 ` [PATCH 6/6] sched: Consider pi boosting in setscheduler Sebastian Andrzej Siewior
2014-02-21 21:32 ` [tip:sched/core] " tip-bot for Thomas Gleixner
2014-02-22 18:02 ` [tip:sched/core] sched: Consider pi boosting in setscheduler() tip-bot for Thomas Gleixner
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