On Tue, Jun 20, 2017 at 01:42:27PM -0400, Rik van Riel wrote: > On Mon, 2017-06-19 at 04:12 +0200, Frederic Weisbecker wrote: > > Although idle load balancing obviously only concern idle CPUs, it can > > be a disturbance on a busy nohz_full CPU. Indeed a CPU can only get > > rid > > of an idle load balancing duty once a tick fires while it runs a task > > and this can take a while in a nohz_full CPU. > > > > We could fix that and escape the idle load balancing duty from the > > very > > idle exit path but that would bring unecessary overhead. Lets just > > not > > bother and leave that job to housekeeping CPUs (those outside > > nohz_full > > range). The nohz_full CPUs simply don't want any disturbance. > > > > Signed-off-by: Frederic Weisbecker <[email protected]> > > Cc: Thomas Gleixner <[email protected]> > > Cc: Ingo Molnar <[email protected]> > > Cc: Rik van Riel <[email protected]> > > Cc: Peter Zijlstra <[email protected]> > > --- > > kernel/sched/fair.c | 4 ++++ > > 1 file changed, 4 insertions(+) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index d711093..cfca960 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -8659,6 +8659,10 @@ void nohz_balance_enter_idle(int cpu) > > if (!cpu_active(cpu)) > > return; > > > > + /* Spare idle load balancing on CPUs that don't want to be > > disturbed */ > > + if (!is_housekeeping_cpu(cpu)) > > + return; > > + > > if (test_bit(NOHZ_TICK_STOPPED, nohz_flags(cpu))) > > return; > > I am not entirely convinced on this one. > > Doesn't the if (on_null_domain(cpu_rq(cpu)) test > a few lines down take care of this already?
It shouldn't, since nohz_full= doesn't imply isolcpus= anymore. Of course it does if the user manually adds them. > > Do we want nohz_full to always automatically > imply that no idle balancing will happen, like > on isolated CPUs? You're making a good point in that I would prefer that nohz_full be only about the tick and let some sort of separate isolation subsystem deal with individual isolation features: nohz, workqueues, idle load balancing, etc... That's why I rather used is_housekeeping_cpu() and not !tick_nohz_full_cpu() because for now housekeepers are ~tick_nohz_full_mask but later it should be cpu_possible_mask by default or some given set of CPUs defined by the future isolation subsystem. Thanks.
