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 -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
