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