From: Steven Rostedt <rostedt@goodmis.org>
To: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@osdl.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: RFC: SysRq nice-all-RT-tasks is broken
Date: Wed, 8 Mar 2017 12:40:12 -0500 [thread overview]
Message-ID: <20170308124012.496a75cf@gandalf.local.home> (raw)
In-Reply-To: <22b0e5fe-2aa1-72de-4d00-29d768d226ca@linux.vnet.ibm.com>
[
Added Peter
Update: Laurent noticed that sysrq 'n' (nice-all-RT-tasks) calls
__sched_setscheduler() form interrupt context. At the start of that
function, there's a BUG_ON(in_interrupt()). The reason for that was
due to the rt mutex pi code calling wait_lock. Which was not irq
safe. Now it is, but that's not good enough.
]
On Wed, 8 Mar 2017 18:03:55 +0100
Laurent Dufour <ldufour@linux.vnet.ibm.com> wrote:
> On 08/03/2017 17:57, Steven Rostedt wrote:
> > On Wed, 8 Mar 2017 11:51:14 -0500
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> >
> >> Hmm, that commit was added in 2.6.18, and you're right, a lot has
> >> changed since then. Have you tried removing it and running it under
> >> lockdep, and see if it triggers any warnings?
> >
> > I did a little digging, and it appears that its the rt mutex wait lock
> > that the comment was referring to. Today that spin lock is irq safe. I
> > believe its safe to remove the BUG_ON(). Want me to send a patch?
>
> Sure, go ahead ;)
>
Actually, it's still not safe :-/
I just noticed this in the call path:
raw_spin_unlock_irq(&task->pi_lock);
As well as other raw_spin_unlock_irq()s.
Which would enable interrupts regardless of the previous state.
One solution is to change all those to irqsave() but that seems to be a
big step for something that is rarely done (how many years has it been
since 2.6.18?).
I wonder if we should just have a special flag sent by that sysrq
trigger. Since it is causing all tasks to go "nice" there's no need to
do the pi chain walk in __sched_setscheduler().
-- Steve
next prev parent reply other threads:[~2017-03-08 17:40 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-08 15:23 RFC: SysRq nice-all-RT-tasks is broken Laurent Dufour
2017-03-08 16:51 ` Steven Rostedt
2017-03-08 16:57 ` Steven Rostedt
2017-03-08 17:03 ` Laurent Dufour
2017-03-08 17:40 ` Steven Rostedt [this message]
2017-03-08 17:46 ` Steven Rostedt
2017-03-09 9:02 ` Laurent Dufour
2017-05-23 8:46 ` [tip:sched/core] sched/core: Allow __sched_setscheduler() in interrupts when PI is not used tip-bot for Steven Rostedt (VMware)
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