Messages in this thread | | | Date | Tue, 15 Dec 2020 15:23:39 +0100 (CET) | From | Anna-Maria Behnsen <> | Subject | Re: [PATCH] sched: Prevent raising SCHED_SOFTIRQ when CPU is !active |
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On Tue, 15 Dec 2020, Peter Zijlstra wrote:
> 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.
As far as I understood, it is not a problem whether the delete is before or after !active. When it is deleted after, the remote CPU will return in kick_ilb() because cpu is not idle, because it is running the hotplug thread.
Thanks,
Anna-Maria
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