Nicholas Piggin <npig...@gmail.com> writes: > On Thu, 3 May 2018 14:36:47 +0530 > Akshay Adiga <akshay.ad...@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.ad...@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.ad...@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.
