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? Do we want nohz_full to always automatically imply that no idle balancing will happen, like on isolated CPUs? -- All rights reversed
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