Re: [Skiboot] [PATCH 1/2] SLW: Remove stop1_lite and stop0 stop states

[Stewart Smith <stewart@linux.vnet.ibm.com>]

Nicholas Piggin <npiggin@gmail.com> writes:
> On Thu, 3 May 2018 14:36:47 +0530
> Akshay Adiga <akshay.adiga@linux.vnet.ibm.com> wrote:
>
>> On Tue, May 01, 2018 at 01:47:23PM +1000, Nicholas Piggin wrote:
>> > On Mon, 30 Apr 2018 14:42:08 +0530
>> > Akshay Adiga <akshay.adiga@linux.vnet.ibm.com> wrote:
>> >   
>> > > Powersaving for stop0_lite and stop1_lite is observed to be quite similar
>> > > and both states resume without state loss. Using context_switch test [1]
>> > > we observe that stop0_lite has slightly lower latency, hence removing
>> > > stop1_lite.
>> > > 
>> > > [1] linux/tools/testing/selftests/powerpc/benchmarks/context_switch.c
>> > > 
>> > > Signed-off-by: Akshay Adiga <akshay.adiga@linux.vnet.ibm.com>  
>> > 
>> > I'm okay for removing stop1_lite and stop2_lite -- SMT switching
>> > is very latency critical. If we decide to actually start saving
>> > real power then SMT should already have been switched.
>> > 
>> > So I would put stop1_lite and stop2_lite removal in the same patch.  
>> 
>> I can do this.
>> 
>> > 
>> > Then what do we have? stop0_lite, stop0, stop1 for our fast idle
>> > states.  
>> 
>> Currently we were looking at  stop0_lite , stop1 as the fast idle states
>> because stop0 and stop1 have similar latency and powersaving.
>> Having so many low latency states does not make sense.
>> 
>> > 
>> > I would be against removing stop0 if that is our fastest way to
>> > release SMT resources, even if there is only a small advantage. Why
>> > not remove stop1 instead?
>> >  
>> SMT-folding comes into picture only when we have at least one thread
>> running in the core. stop0 and stop1 has exactly same power-saving and
>> both will release SMT resources if at least one thread in the core is
>> running.
>
> Right, but you don't know that other threads are running or will remain
> running when you enter stop. If not, then latency is higher for stop1,
> no? So we need to be using stop0.
>
>> 
>> As soon as all threads are idle core enters stop0/stop1, where stop1
>> does a bit more powersaving than stop0.
>> 
>> > We also need to better evaluate stop0_lite. How much advantage does
>> > that have over snooze?  
>> 
>> I evaluated snooze and stop0_lite, there is an additional ipi latency of
>> a few microseconds in case of stop0_lite. So snooze cannot still be
>> replaced by stop0_lite.
>
> I meant the other way around. Replace stop0_lite with snooze.
>
> So we would have snooze, stop0, stop2, and stop4 and/or 5.

Slightly stupid question: should we be disabling these here or should
Linux be better and deciding what states to use?

I'm inclined to say this is a Linux problem as it should make the
decision of what hardware feature to used based on the ones OPAL says
*can* be used.

I'm also open to be being convinced otherwise though...

-- 
Stewart Smith
OPAL Architect, IBM.