linux-kernel - Re: [PATCH RFC] pm_qos

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Date:	Wed, 6 Aug 2008 00:18:08 +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 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

View attachment "pm_qos_requirement.patch" of type "text/x-patch" (1640 bytes)