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Subject: Re: [PATCH RFC] pm_qos_requirement might sleep
From: Peter Zijlstra <peterz@infradead.org>
To: mgross@linux.intel.com
Cc: John Kacur <jkacur@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
       rt-users <linux-rt-users@vger.kernel.org>,
       Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>, arjan <arjan@infradead.org>
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	 <1217921101.3589.98.camel@twins>  <20080805204901.GA31132@linux.intel.com>
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Date: Tue, 05 Aug 2008 23:09:46 +0200
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On Tue, 2008-08-05 at 13:49 -0700, mark gross wrote:
> On Tue, Aug 05, 2008 at 09:25:01AM +0200, Peter Zijlstra wrote:
> > On Mon, 2008-08-04 at 22:52 +0200, John Kacur wrote:
> > > Even after applying some fixes posted by Chirag and Peter Z, I'm still
> > > getting some messages in my log like this
> > 
> > > BUG: sleeping function called from invalid context swapper(0) at
> > > kernel/rtmutex.c:743
> > > in_atomic():1 [00000001], irqs_disabled():1
> > > Pid: 0, comm: swapper Tainted: G        W 2.6.26.1-rt1.jk #2
> > > 
> > > Call Trace:
> > >  [<ffffffff802305d3>] __might_sleep+0x12d/0x132
> > >  [<ffffffff8046cdbe>] __rt_spin_lock+0x34/0x7d
> > >  [<ffffffff8046ce15>] rt_spin_lock+0xe/0x10
> > >  [<ffffffff802532e5>] pm_qos_requirement+0x1f/0x3c
> > >  [<ffffffff803e1b7f>] menu_select+0x7b/0x9c
> > >  [<ffffffff8020b1be>] ? default_idle+0x0/0x5a
> > >  [<ffffffff8020b1be>] ? default_idle+0x0/0x5a
> > >  [<ffffffff803e0b4b>] cpuidle_idle_call+0x68/0xd8
> > >  [<ffffffff803e0ae3>] ? cpuidle_idle_call+0x0/0xd8
> > >  [<ffffffff8020b1be>] ? default_idle+0x0/0x5a
> > >  [<ffffffff8020b333>] cpu_idle+0xb2/0x12d
> > >  [<ffffffff80466af0>] start_secondary+0x186/0x18b
> > > 
> > > ---------------------------
> > > | preempt count: 00000001 ]
> > > | 1-level deep critical section nesting:
> > > ----------------------------------------
> > > ... [<ffffffff8020b39c>] .... cpu_idle+0x11b/0x12d
> > > ......[<ffffffff80466af0>] ..   ( <= start_secondary+0x186/0x18b)
> > > 
> > > The following simple patch makes the messages disappear - however,
> > > there may be a better more fine grained solution, but the problem is
> > > also that all the functions are designed to use the same lock.
> > 
> > Hmm, I think you're right - its called from the idle routine so we can't
> > go about sleeping there.
> > 
> > The only trouble I have is with kernel/pm_qos_params.c:update_target()'s
> > use of this lock - that is decidedly not O(1).
> > 
> > Mark, would it be possible to split that lock in two, one lock
> > protecting pm_qos_array[], and one lock protecting the
> > requirements.list ?
> 
> very likely, but I'm not sure how it will help.
> 
> the fine grain locking I had initially worked out on pm_qos was to have
> a lock per pm_qos_object, that would be used for accessing the
> requirement_list and the target_value.  But that isn't what you are
> asking about is it?
> 
> Is what you want is a pm_qos_requirements_list_lock and a
> pm_qos_target_value_lock, for each pm_qos_object instance?
> 
> I guess it wold work but besides giving the code spinlock diarrhea would
> it really help solve the issue you are seeing? 

The problem is that on -rt spinlocks turn into mutexes. And the above
BUG tells us that the idle loop might end up scheduling due to trying to
take this lock.

Now, the way I read the code, pm_qos_lock protects multiple things:

 - pm_qos_array[target]->target_value

 - &pm_qos_array[pm_qos_class]->requirements.list

Now, the thing is, we could turn the lock back into a real spinlock
(raw_spinlock_t), but the loops in eg update_target() are not O(1) and
could thus cause serious preempt-off latencies.

My question was, and now having had a second look at the code I think it
is, would it be possible to guard the list using a sleeping lock,
protect the target_value using a (raw) spinlock.

OTOH, just reading a (word aligned, word sized) value doesn't normally
require serialization, esp if the update site is already serialized by
other means.

So could we perhaps remove the lock usage from pm_qos_requirement()? -
that too would solve the issue.


 - Peter

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