Re: [PATCH 3/3] PM: Introduce Intel PowerClamp Driver

[Arjan van de Ven <arjan@linux.intel.com>]

On 11/13/2012 2:23 PM, Paul E. McKenney wrote:
> On Tue, Nov 13, 2012 at 01:39:22PM -0800, Jacob Pan wrote:
>> On Tue, 13 Nov 2012 13:16:02 -0800
>> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>>
>>>> Please refer to Documentation/thermal/intel_powerclamp.txt for more
>>>> details.  
>>>
>>> If I read this correctly, this forces a group of CPUs into idle for
>>> about 600 milliseconds at a time.  This would indeed delay grace
>>> periods, which could easily result in user complaints.  Also, given
>>> the default RCU_BOOST_DELAY of 500 milliseconds in kernels enabling
>>> RCU_BOOST, you would see needless RCU priority boosting.
>>>
>> the default idle injection duration is 6ms. we adjust the sleep
>> interval to ensure idle ratio. So the idle duration stays the same once
>> set. So would it be safe to delay grace period for this small amount in
>> exchange for less over head in each injection period?
> 
> Ah, 6ms of delay is much better than 600ms.  Should be OK (famous last
> words!).

well... power clamping is not "free".
You're going to lose performance as a trade off for dropping instantaneous power consumption....
in the measurements we've done comparing various methods.. this one is doing remarkably well.

> 
> For most kernel configuration options, it does use softirq.  And yes,
> the kthread you are using would yield to softirqs -- but only as long
> as softirq processing hasn't moved over to ksoftirqd.  Longer term,
> RCU will be moving from softirq to kthreads, though, and these might be
> prempted by your powerclamp kthread, depending on priorities.  It looks
> like you use RT prio 50, which would usually preempt the RCU kthreads
> (unless someone changed the priorities).

we tried to pick a "middle of the road" value, so that usages that really really
want to run, still get to run, but things that are more loose about it, get put on hold.


> 
>>> It looks like you could end up with part of the system powerclamped
>>> in some situations, and with all of it powerclamped in other
>>> situations. Is that the case, or am I confused?
>>>
>> could you explain the part that is partially powerclamped?
> 
> Suppose that a given system has two sockets.  Are the two sockets
> powerclamped independently, or at the same time?  My guess was the
> former, but looking at the code again, it looks like the latter.
> So it is a good thing I asked, I guess.  ;-)

they are clamped together, and they have to.
you don't get (on the systems where this driver works) any "package" C state unless
all packages are idle completely.
And it's these package C states where the real deep power savings happen, that's
why they are such a juicy target for power clamping ;-)