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.
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.
>
> Thanks,
> Nick
>
>
> > ---
> > hw/slw.c | 30 ------------------------------
> > 1 file changed, 30 deletions(-)
> >
> > diff --git a/hw/slw.c b/hw/slw.c
> > index 3f9abaa..edfc783 100644
> > --- a/hw/slw.c
> > +++ b/hw/slw.c
> > @@ -521,36 +521,6 @@ static struct cpu_idle_states power9_cpu_idle_states[]
> > = {
> > | OPAL_PM_PSSCR_TR(3),
> > .pm_ctrl_reg_mask = OPAL_PM_PSSCR_MASK },
> > {
> > - .name = "stop0",
> > - .latency_ns = 2000,
> > - .residency_ns = 20000,
> > - .flags = 0*OPAL_PM_DEC_STOP \
> > - | 0*OPAL_PM_TIMEBASE_STOP \
> > - | 1*OPAL_PM_LOSE_USER_CONTEXT \
> > - | 0*OPAL_PM_LOSE_HYP_CONTEXT \
> > - | 0*OPAL_PM_LOSE_FULL_CONTEXT \
> > - | 1*OPAL_PM_STOP_INST_FAST,
> > - .pm_ctrl_reg_val = OPAL_PM_PSSCR_RL(0) \
> > - | OPAL_PM_PSSCR_MTL(3) \
> > - | OPAL_PM_PSSCR_TR(3) \
> > - | OPAL_PM_PSSCR_ESL \
> > - | OPAL_PM_PSSCR_EC,
> > - .pm_ctrl_reg_mask = OPAL_PM_PSSCR_MASK },
> > - {
> > - .name = "stop1_lite", /* Enter stop1 with no state loss */
> > - .latency_ns = 4900,
> > - .residency_ns = 49000,
> > - .flags = 0*OPAL_PM_DEC_STOP \
> > - | 0*OPAL_PM_TIMEBASE_STOP \
> > - | 0*OPAL_PM_LOSE_USER_CONTEXT \
> > - | 0*OPAL_PM_LOSE_HYP_CONTEXT \
> > - | 0*OPAL_PM_LOSE_FULL_CONTEXT \
> > - | 1*OPAL_PM_STOP_INST_FAST,
> > - .pm_ctrl_reg_val = OPAL_PM_PSSCR_RL(1) \
> > - | OPAL_PM_PSSCR_MTL(3) \
> > - | OPAL_PM_PSSCR_TR(3),
> > - .pm_ctrl_reg_mask = OPAL_PM_PSSCR_MASK },
> > - {
> > .name = "stop1",
> > .latency_ns = 5000,
> > .residency_ns = 50000,
>
