Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

[Li, Aubrey]

On 2017/7/13 22:53, Peter Zijlstra wrote:
> On Thu, Jul 13, 2017 at 10:48:55PM +0800, Li, Aubrey wrote:
> 
>> - totally from arch_cpu_idle_enter entry to arch_cpu_idle_exit return costs
>>   9122ns - 15318ns.
>> ---- In this period(arch idle), rcu_idle_enter costs 1985ns - 2262ns, rcu_idle_exit
>>      costs 1813ns - 3507ns
>>
>> Besides RCU,
> 
> So Paul wants more details on where RCU hurts so we can try to fix.
>
If we can call RCU idle enter/exit after tick is really stopped, instead of
call it every idle, I think it's fine. Then we can skip stopping tick if we need
fast idle.
 
>> the period includes c-state selection on X86, a few timestamp updates
>> and a few computations in menu governor. Also, deep HW-cstate latency can be up
>> to 100+ microseconds, even if the system is very busy, CPU still has chance to enter
>> deep cstate, which I guess some outburst workloads are not happy with it.
>>
>> That's my major concern without a fast idle path.
> 
> Fixing C-state selection by creating an alternative idle path sounds so
> very wrong.

This only happens on the arch which has multiple hardware idle cstates, like
Intel's processor. As long as we want to support multiple cstates, we have to
make a selection(with cost of timestamp update and computation). That's fine
in the normal idle path, but if we want a fast idle switch, we can make a 
tradeoff to use a low-latency one directly, that's why I proposed a fast idle
path, so that we don't need to mix fast idle condition judgement in both idle
entry and idle exit path.

Thanks,
-Aubrey