On Fri, Jul 14, 2017 at 11:56:33AM +0800, Li, Aubrey wrote: > On 2017/7/14 2:28, Peter Zijlstra wrote: > > On Thu, Jul 13, 2017 at 11:13:28PM +0800, Li, Aubrey wrote: > >> On 2017/7/13 22:53, Peter Zijlstra wrote: > > > >>> 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. > > > > That doesn't make sense. If you can decide to pick a shallow C state in > > any way, you can fix the general selection too. > > > > Okay, maybe something like the following make sense? Give a hint to > cpuidle_idle_call() to indicate a fast idle. > > -------------------------------------------------------- > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > index ef63adc..3165e99 100644 > --- a/kernel/sched/idle.c > +++ b/kernel/sched/idle.c > @@ -152,7 +152,7 @@ static void cpuidle_idle_call(void) > */ > rcu_idle_enter(); > > - if (cpuidle_not_available(drv, dev)) { > + if (cpuidle_not_available(drv, dev) || this_is_a_fast_idle) { > default_idle_call(); > goto exit_idle; > }
No, that's wrong. We want to fix the normal C state selection process to pick the right C state. The fast-idle criteria could cut off a whole bunch of available C states. We need to understand why our current C state pick is wrong and amend the algorithm to do better. Not just bolt something on the side.