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    SubjectRe: [PATCH RFC] pm_qos_requirement might sleep
    On Wed, Aug 06, 2008 at 12:18:08AM +0200, John Kacur wrote:
    > On Tue, Aug 5, 2008 at 11:09 PM, Peter Zijlstra <peterz@infradead.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

    > pm_qos_requirement-fix
    > Signed-off-by: John Kacur <jkacur at gmail dot com>
    >
    > Add a raw spinlock for the target value.
    >
    >
    > Index: linux-2.6.26.1-rt1.jk/kernel/pm_qos_params.c
    > ===================================================================
    > --- linux-2.6.26.1-rt1.jk.orig/kernel/pm_qos_params.c
    > +++ linux-2.6.26.1-rt1.jk/kernel/pm_qos_params.c
    > @@ -111,6 +111,7 @@ static struct pm_qos_object *pm_qos_arra
    > };
    >
    > static DEFINE_SPINLOCK(pm_qos_lock);
    > +static DEFINE_RAW_SPINLOCK(pm_qos_rawlock);
    >
    > static ssize_t pm_qos_power_write(struct file *filp, const char __user *buf,
    > size_t count, loff_t *f_pos);
    > @@ -149,13 +150,15 @@ static void update_target(int target)
    > extreme_value = pm_qos_array[target]->comparitor(
    > extreme_value, node->value);
    > }
    > + spin_unlock_irqrestore(&pm_qos_lock, flags);
    > + spin_lock_irqsave(&pm_qos_rawlock, flags);
    > if (pm_qos_array[target]->target_value != extreme_value) {
    > call_notifier = 1;
    > pm_qos_array[target]->target_value = extreme_value;
    > pr_debug(KERN_ERR "new target for qos %d is %d\n", target,
    > pm_qos_array[target]->target_value);
    > }
    > - spin_unlock_irqrestore(&pm_qos_lock, flags);
    > + spin_unlock_irqrestore(&pm_qos_rawlock, flags);
    >
    > if (call_notifier)
    > blocking_notifier_call_chain(pm_qos_array[target]->notifiers,
    > @@ -195,9 +198,9 @@ int pm_qos_requirement(int pm_qos_class)
    > int ret_val;
    > unsigned long flags;
    >
    > - spin_lock_irqsave(&pm_qos_lock, flags);
    > + spin_lock_irqsave(&pm_qos_rawlock, flags);
    > ret_val = pm_qos_array[pm_qos_class]->target_value;
    > - spin_unlock_irqrestore(&pm_qos_lock, flags);
    > + spin_unlock_irqrestore(&pm_qos_rawlock, flags);
    >
    > return ret_val;
    > }

    As long as RAW_SPINLOCK compiles to a normal spinlock for non-RT premept
    kernels I'm don't see a problem, as the change is almost a no-op for
    non-RT kernels.

    Signed-off-by: mark gross <mgross@linux.intel.com>

    Should I send an updated patch that includes a change to the comment
    block regarding the locking design after this patch or instead of it?


    --gmross




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