On Wed, Aug 6, 2008 at 12:18 AM, John Kacur <jkacur@xxxxxxxxx> wrote: > On Tue, Aug 5, 2008 at 11:09 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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. -- 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
