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

On Tue, May 01, 2018 at 01:47:23PM +1000, Nicholas Piggin wrote:

On Mon, 30 Apr 2018 14:42:08 +0530
Akshay Adiga [off-list ref] wrote:

quoted

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 <redacted>

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

quoted

---
 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,