[Skiboot] [PATCH v2] SLW: Remove stop1_lite and stop2_lite

[Balbir Singh]

On 25/05/18 14:36, Vaidyanathan Srinivasan wrote:
> * Balbir Singh <bsingharora at gmail.com> [2018-05-23 21:37:44]:
> 
>> On Wed, May 23, 2018 at 6:17 PM, Akshay Adiga
>> <akshay.adiga at linux.vnet.ibm.com> wrote:
>>> stop1_lite has been removed since it adds no additional benefit
>>> over stop0_lite. stop2_lite has been removed since currently it adds
>>> minimal benefit over stop2. However, the benefit is eclipsed by the time
>>> required to ungate the clocks
>>>
>>> Moreover, Lite states don't give up the SMT resources, can potentially
>>> have a performance impact on sibling threads.
>>>
>>> Signed-off-by: Akshay Adiga <akshay.adiga at linux.vnet.ibm.com>
>>> ---
>>>  hw/slw.c | 36 ++++++++----------------------------
>>>  1 file changed, 8 insertions(+), 28 deletions(-)
>>>
>>> diff --git a/hw/slw.c b/hw/slw.c
>>> index 2b305db..ff6e2a4 100644
>>> --- a/hw/slw.c
>>> +++ b/hw/slw.c
>>> @@ -529,20 +529,9 @@ static struct cpu_idle_states power9_cpu_idle_states[] = {
>>>                                  | 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,
>>
>> How did we come up with the latency and residency values for
>> stop1_lite, stop2_lite? It sounds like the values are roughtly 2% of
>> the corresponding non lite states? Are these values empirical,
>> guesses, retrofitted to the OS?
> 
> These values are based on real latency, but bumped up to fit into
> kernel with sufficient space among other stop states to make an
> independent selection. It is a mix of measurements and heuristics.
> This patch reduces the number of stop states so that we keep the ones
> that make a difference. This will help us tune these number better.
> 

OK, so the next question is why did we take out the stop lite states and not
the stop states? Is there a huge saving with the actual stop states compared
to the lite states? IOW, can we get similar numbers without the state loss?

Balbir Singh.