linux-kernel - Re: [PATCH RFC] pm_qos

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Date:	Mon, 11 Aug 2008 15:25:49 +0200
From:	"John Kacur" <jkacur@...il.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	mgross@...ux.intel.com, LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>, arjan <arjan@...radead.org>
Subject: Re: [PATCH RFC] pm_qos_requirement might sleep

On Wed, Aug 6, 2008 at 12:18 AM, John Kacur <jkacur@...il.com> wrote:
> On Tue, Aug 5, 2008 at 11:09 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> 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
>>
>
> How about this patch? Like Peter suggests, It adds a raw spinlock only
> for the target value. I'm currently running with it, but still
> testing, comments are appreciated.
>
> Thanks
>

I have been running with this patch for quite a while now without any
problems, so please apply. If Mark is able to remove the lock
altogether at some point in the future then we can remove this patch.
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