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Date: Tue, 5 Aug 2008 13:49:01 -0700
From: mark gross <mgross@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
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>
Subject: Re: [PATCH RFC] pm_qos_requirement might sleep
Message-ID: <20080805204901.GA31132@linux.intel.com>
Reply-To: mgross@linux.intel.com
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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? 

--mgross

 
> [ NOTE: this is the -rt kernel we're talking about ]
> 
> > Signed-off-by: John Kacur <jkacur at gmail dot com>
> > 
> > Index: linux-2.6.26.1-jk-rt1/kernel/pm_qos_params.c
> > ===================================================================
> > --- linux-2.6.26.1-jk-rt1.orig/kernel/pm_qos_params.c
> > +++ linux-2.6.26.1-jk-rt1/kernel/pm_qos_params.c
> > @@ -110,7 +110,7 @@ static struct pm_qos_object *pm_qos_arra
> >         &network_throughput_pm_qos
> >  };
> >  
> > -static DEFINE_SPINLOCK(pm_qos_lock);
> > +static DEFINE_RAW_SPINLOCK(pm_qos_lock);
> >  
> >  static ssize_t pm_qos_power_write(struct file *filp, const char
> > __user *buf,
> >                 size_t count, loff_t *f_pos);
> > 
--
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