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

[Peter Zijlstra]

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@xxxxxxxxxxxxx>
> ---
>  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!