Re: [PATCH] sched: Prevent raising SCHED_SOFTIRQ when CPU is !active

[Anna-Maria Behnsen <anna-maria@linutronix.de>]

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