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

From: Peter Zijlstra
Date: Thu Jul 13 2017 - 10:53:31 EST

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.

> 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.