Date:	Tue, 26 Aug 2008 19:45:37 +0200
From:	"John Kacur" <jkacur@...il.com>
To:	mgross@...ux.intel.com
Cc:	LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	"Peter Zijlstra" <peterz@...radead.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 26, 2008 at 6:18 PM, mark gross <mgross@...ux.intel.com> wrote:
> On Tue, Aug 26, 2008 at 10:48:13AM +0200, John Kacur wrote:
>> On Mon, Aug 25, 2008 at 6:35 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Mon, 2008-08-25 at 09:34 -0700, mark gross wrote:
>> >> On Fri, Aug 15, 2008 at 12:51:11AM +0200, John Kacur wrote:
>> >> > On Thu, Aug 14, 2008 at 7:48 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> >> > > On Thu, 2008-08-14 at 08:52 -0700, mark gross wrote:
>> >> > >
>> >> > >> Keeping a lock around the different "target_value"s may not be so
>> >> > >> important.  Its just a 32bit scaler value, and perhaps we can make it an
>> >> > >> atomic type?  That way we loose the raw_spinlock.
>> >> > >
>> >> > > My suggestion was to keep the locking for the write side - so as to
>> >> > > avoid stuff stomping on one another, but drop the read side as:
>> >> > >
>> >> > >  spin_lock
>> >> > >  foo = var;
>> >> > >  spin_unlock
>> >> > >  return foo;
>> >> > >
>> >> > > is kinda useless, it doesn't actually serialize against the usage of
>> >> > > foo, that is, once it gets used, var might already have acquired a new
>> >> > > value.
>> >> > >
>> >> > > The only thing it would protect is reading var, but since that is a
>> >> > > machine sized read, its atomic anyway (assuming its naturally aligned).
>> >> > >
>> >> > > So no need for atomic_t (its read-side is just a read too), just drop
>> >> > > the whole lock usage from pq_qos_requirement().
>> >> > >
>> >> >
>> >> > Thanks Peter.
>> >> >
>> >> > Mark, is the following patch ok with you? This should be applied to
>> >> > mainline, and then after that no special patches are necessary for
>> >> > real-time.
>> >>
>> >> I've been thinking about this patch and I worry that the readability
>> >> from making the use of this lock asymmetric WRT reads and writes to the
>> >> storage address is bothersome.
>> >>
>> >> I would rather make the variable an atomic.  What do you think about
>> >> that?
>> >
>> > It would make the write side more expensive, as we already have the two
>> > atomic operations for the lock and unlock, this would add a third.
>> >
>> > Then again, I doubt that this is really a fast path.
>> >
>> > OTOH, a simple comment could clarify the situation for the reader.
>> >
>> > Up to you I guess ;-)
>> >
>>
>> Personally I agree with Peter, a simple comment would clarify the
>> situation, it seems quite silly to me to add complexity in the name of
>> symmetry. This is not my definition of readability. Never-the-less I
>> offer up solution number 3 here if that would please everyone more.
>> Attached is a patch that changes the target value to an atomic
>> variable as suggested by Arjan. To summarize.
>>
>> 3 Sol'ns - all of which solve the problem.
>> 1. Add a raw spinlock around target value only. This makes the raw
>> spinlock area very small, and is converted to a normal spinlock for
>> non-preempt-rt.
>> 2. Remove the spinlock altogether in pm_qos_requirement since the
>> simple read is already atomic. Advantage - smallest patch and realtime
>> doesn't require a special patch once this is included in mainline. I
>> like this one the best.
>> 3. make target_value atomic_t. Advantage - symmetry, some people find
>> this more readable. The patch is larger than the above solution but as
>> above, no special patch is required for realtime once this is included
>> in mainline. Solution three is in the attached patch. Comments are
>> appreciated as always.
>
> Thank you!  FWIW I'm really on the fence between option 2 and 3.
>
>> Remove the spinlock in pm_qos_requirement by making target_value an atomic type.
>> This is necessary for real-time since pm_qos_requirement is called by idle and
>> cannot be allowed to sleep.
>> Signed-off-by: John Kacur <jkacur at gmail dot com>
>>
>> Index: linux-2.6.26.3-rt3/kernel/pm_qos_params.c
>> ===================================================================
>> --- linux-2.6.26.3-rt3.orig/kernel/pm_qos_params.c
>> +++ linux-2.6.26.3-rt3/kernel/pm_qos_params.c
>> @@ -42,7 +42,7 @@
>>  #include <linux/uaccess.h>
>>
>>  /*
>> - * locking rule: all changes to target_value or requirements or notifiers lists
>> + * locking rule: all changes to requirements or notifiers lists
>>   * or pm_qos_object list and pm_qos_objects need to happen with pm_qos_lock
>>   * held, taken with _irqsave.  One lock to rule them all
>>   */
>> @@ -65,7 +65,7 @@ struct pm_qos_object {
>>       struct miscdevice pm_qos_power_miscdev;
>>       char *name;
>>       s32 default_value;
>> -     s32 target_value;
>> +     atomic_t target_value;
>>       s32 (*comparitor)(s32, s32);
>>  };
>>
>> @@ -76,7 +76,7 @@ static struct pm_qos_object cpu_dma_pm_q
>>       .notifiers = &cpu_dma_lat_notifier,
>>       .name = "cpu_dma_latency",
>>       .default_value = 2000 * USEC_PER_SEC,
>> -     .target_value = 2000 * USEC_PER_SEC,
>> +     .target_value = ATOMIC_INIT(2000 * USEC_PER_SEC),
>>       .comparitor = min_compare
>>  };
>>
>> @@ -86,7 +86,7 @@ static struct pm_qos_object network_lat_
>>       .notifiers = &network_lat_notifier,
>>       .name = "network_latency",
>>       .default_value = 2000 * USEC_PER_SEC,
>> -     .target_value = 2000 * USEC_PER_SEC,
>> +     .target_value = ATOMIC_INIT(2000 * USEC_PER_SEC),
>>       .comparitor = min_compare
>>  };
>>
>> @@ -98,7 +98,7 @@ static struct pm_qos_object network_thro
>>       .notifiers = &network_throughput_notifier,
>>       .name = "network_throughput",
>>       .default_value = 0,
>> -     .target_value = 0,
>> +     .target_value = ATOMIC_INIT(0),
>>       .comparitor = max_compare
>>  };
>>
>> @@ -149,13 +149,14 @@ static void update_target(int target)
>>               extreme_value = pm_qos_array[target]->comparitor(
>>                               extreme_value, node->value);
>>       }
>> -     if (pm_qos_array[target]->target_value != extreme_value) {
>> +     spin_unlock_irqrestore(&pm_qos_lock, flags);
>> +
>
> do we want to move the unlock before the setting of the target_value?
> This feels wrong to me, the option 2 patch didn't do this.
>
> couldn't we have a race from 2 cpu's hitting update_target at the same
> time with different values if we drop the lock before the target_value
> is set?

I think you are right since atomicity doesn't have anything to do with
ordering, good catch, putting the the unlock back where it was before,
new patch attached. (also shortened-up pm_qos_requirement)

---SNIP----

John

View attachment "pm_qos_requirement.patch" of type "text/x-patch" (2889 bytes)

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