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

Paul E. McKenney
2017-07-19T14:48:37Z

On Wed, Jul 19, 2017 at 01:44:06PM +0800, Li, Aubrey wrote:
> On 2017/7/18 23:20, Paul E. McKenney wrote:
>
> >> 2) for rcu idle enter/exit, I measured the details which Paul provided, and
> >> the result matches with what I have measured before, nothing notable found.
> >> But it still makes more sense if we can make rcu idle enter/exit hooked with
> >> tick off. (it's possible other workloads behave differently)
> >
> > Again, assuming that RCU is informed of CPUs in the kernel, regardless
> > of whether or not the tick is on that that point in time.
> >
> Yeah, I see, no problem for a normal idle.
>
> But for a short idle, we want to return to the task ASAP. Even though RCU cost
> is not notable, it would still be better for me if we can save some cycles in
> idle entry and idle exit.
>
> Do we have any problem if we skip RCU idle enter/exit under a fast idle scenario?
> My understanding is, if tick is not stopped, then we don't need inform RCU in
> idle path, it can be informed in irq exit.

Indeed, the problem arises when the tick is stopped.

Thanx, Paul