linux-kernel - Re: [tip:sched/core] sched: Change the sched_class::set_cpus

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Date:	Thu, 13 Aug 2015 23:30:03 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	hpa@...or.com, torvalds@...ux-foundation.org, efault@....de,
	tglx@...utronix.de, linux-kernel@...r.kernel.org, mingo@...nel.org,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Change the
 sched_class::set_cpus_allowed( ) calling context

On Thu, Aug 13, 2015 at 04:59:01PM -0400, Sasha Levin wrote:

> Seems to work fine now, thanks!

I've no clue how this doesn't also explode on actual hardware, that code
is convoluted, read and weep.

Ingo, please stick somewhere appropriate :-)

---
Subject: sched: Avoid trying to dequeue/enqueue the idle thread
From: Peter Zijlstra <peterz@...radead.org>
Date: Thu Aug 13 23:09:29 CEST 2015

Sasha reports that his virtual machine tries to schedule the idle
thread since commit 6c37067e2786 ("sched: Change the
sched_class::set_cpus_allowed() calling context").

His trace shows this happening from idle_thread_get()->init_idle(),
which is the _second_ init_idle() invocation on that task_struct, the
first being done through idle_init()->fork_idle(). (this code is
insane...)

Because we call init_idle() twice in a row, its ->sched_class ==
&idle_sched_class and ->on_rq = TASK_ON_RQ_QUEUED. This means
do_set_cpus_allowed() thinks we're queued and will call dequeue_task(),
which is implemented with BUG() for the idle class, seeing how
dequeueing the idle task is a daft thing.

Aside of the whole insanity of calling init_idle() _twice_, change the
code to call set_cpus_allowed_common() instead as this is 'obviously'
before the idle task gets ran etc..

Fixes: 6c37067e2786 ("sched: Change the sched_class::set_cpus_allowed() calling context")
Reported-by: Sasha Levin <sasha.levin@...cle.com>
Tested-by: Sasha Levin <sasha.levin@...cle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/core.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4924,7 +4924,7 @@ void init_idle(struct task_struct *idle,
 	idle->state = TASK_RUNNING;
 	idle->se.exec_start = sched_clock();

-	do_set_cpus_allowed(idle, cpumask_of(cpu));
+	set_cpus_allowed_common(idle, cpumask_of(cpu));
 	/*
 	 * We're having a chicken and egg problem, even though we are
 	 * holding rq->lock, the cpu isn't yet set to this cpu so the
--
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