Messages in this thread | | | Date | Wed, 26 Jun 2013 15:24:42 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC nohz_full 0/8] Provide infrastructure for full-system idle |
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On Wed, Jun 26, 2013 at 02:20:22PM +0200, Peter Zijlstra wrote: > On Tue, Jun 25, 2013 at 02:37:21PM -0700, Paul E. McKenney wrote: > > Whenever there is at least one non-idle CPU, it is necessary to > > periodically update timekeeping information. Before NO_HZ_FULL, this > > updating was carried out by the scheduling-clock tick, which ran on > > every non-idle CPU. With the advent of NO_HZ_FULL, it is possible > > to have non-idle CPUs that are not receiving scheduling-clock ticks. > > This possibility is handled by assigning a timekeeping CPU that continues > > taking scheduling-clock ticks. > > > > Unfortunately, timekeeping CPU continues taking scheduling-clock > > interrupts even when all other CPUs are completely idle, which is > > not so good for energy efficiency and battery lifetime. Clearly, it > > would be good to turn off the timekeeping CPU's scheduling-clock tick > > when all CPUs are completely idle. This is conceptually simple, but > > we also need good performance and scalability on large systems, which > > rules out implementations based on frequently updated global counts of > > non-idle CPUs as well as implementations that frequently scan all CPUs. > > Nevertheless, we need a single global indicator in order to keep the > > overhead of checking acceptably low. > > > > The chosen approach is to enforce hysteresis on the non-idle to > > full-system-idle transition, with the amount of hysteresis increasing > > linearly with the number of CPUs, thus keeping contention acceptably low. > > This approach piggybacks on RCU's existing force-quiescent-state scanning > > of idle CPUs, which has the advantage of avoiding the scan entirely on > > busy systems that have high levels of multiprogramming. This scan > > take per-CPU idleness information and feeds it into a state machine > > that applies the level of hysteresis required to arrive at a single > > full-system-idle indicator. > > > > Note that this version pays attention to CPUs that have taken an NMI > > from idle. It is not clear to me that NMI handlers can safely access > > the time on a system that is long-term idle. Unless someone tells me > > that it is somehow safe to access time from an NMI from idle, I will > > remove NMI support in the next version. > > Using perf it is 'possible' to come near; we use local_clock() from NMI > context. It will do a TSC read. > > On systems where the TSC is usable we'll end up with a sane timestamp; > on systems where we need the whole kernel/sched/clock.c song and dance > routine we'll return a stable time-stamp when called from long idle. > > I don't think there's anything we can do better there.
Just to make sure I understand... You are saying that it is OK for NO_HZ_FULL to shut down timekeeping if all CPUs are idle, even if some of them are taking NMIs from time to time, right?
Thanx, Paul
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