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    SubjectRe: [PATCH RFC] pm_qos_requirement might sleep
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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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