Messages in this thread | | | Date | Tue, 15 Dec 2020 12:18:06 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: Prevent raising SCHED_SOFTIRQ when CPU is !active |
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On Tue, Dec 15, 2020 at 11:44:00AM +0100, Anna-Maria Behnsen wrote: > SCHED_SOFTIRQ is raised to trigger periodic load balancing. When CPU is not > active, CPU should not participate in load balancing. > > The scheduler uses nohz.idle_cpus_mask to keep track of the CPUs which can > do idle load balancing. When bringing a CPU up the CPU is added to the mask > when it reaches the active state, but on teardown the CPU stays in the mask > until it goes offline and invokes sched_cpu_dying(). > > When SCHED_SOFTIRQ is raised on a !active CPU, there might be a pending > softirq when stopping the tick which triggers a warning in NOHZ code. The > SCHED_SOFTIRQ can also be raised by the scheduler tick which has the same > issue. > > Therefore remove the CPU from nohz.idle_cpus_mask when it is marked > inactive and also prevent the scheduler_tick() from raising SCHED_SOFTIRQ > after this point. > > Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> > --- > kernel/sched/core.c | 7 ++++++- > kernel/sched/fair.c | 7 +++++-- > 2 files changed, 11 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 21b548b69455..69284dc121d3 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -7492,6 +7492,12 @@ int sched_cpu_deactivate(unsigned int cpu) > struct rq_flags rf; > int ret; > > + /* > + * Remove CPU from nohz.idle_cpus_mask to prevent participating in > + * load balancing when not active > + */ > + nohz_balance_exit_idle(rq); > + > set_cpu_active(cpu, false); > /* > * We've cleared cpu_active_mask, wait for all preempt-disabled and RCU
OK, so we must clear the state before !active, because getting an interrupt/softirq after would trigger the badness. And we're guaranteed nothing blocks between them to re-set it.
That works I suppose.
Thanks!
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