Re: [PATCH] sched, fair: try to prevent migration thread from preempting non-cfs task

[Peter Zijlstra]

On Tue, Jun 15, 2021 at 08:15:51PM +0800, Yafang Shao wrote:
> ---
>  kernel/sched/fair.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3248e24a90b0..597c7a940746 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9797,6 +9797,20 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>  			/* Record that we found at least one task that could run on this_cpu */
>  			env.flags &= ~LBF_ALL_PINNED;
>  
> +			/*
> +			 * There may be a race between load balance starting migration
> +			 * thread to pull the cfs running thread and the RT thread
> +			 * waking up and preempting cfs task before migration threads
> +			 * which then preempt the RT thread.
> +			 * We'd better do the last minute check before starting
> +			 * migration thread to avoid preempting latency-sensitive thread.
> +			 */
> +			if (busiest->curr->sched_class != &fair_sched_class) {
> +				raw_spin_unlock_irqrestore(&busiest->lock,
> +							   flags);

This won't apply.

Also, there's still a race window, you've just shrunk it, not fixed it.
Busiest can reschedule between the mandatory rq unlock and doing the
stopper wakeup.

An actual fix might be to have the active migration done by a FIFO-1
task, instead of stopper. The obvious down-side is that that would mean
spawning yet another per-cpu kthread.

How much do we care?