Re: [PATCH RFC nohz_full 0/8] Provide infrastructure for full-system idle

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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