Re: [RFC PATCH 03/13] sched: Core-wide rq->lock

[Peter Zijlstra]

On Wed, Mar 04, 2020 at 04:59:53PM +0000, vpillai wrote:
> @@ -6400,8 +6464,15 @@ int sched_cpu_activate(unsigned int cpu)
>  	/*
>  	 * When going up, increment the number of cores with SMT present.
>  	 */
> -	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
> +	if (cpumask_weight(cpu_smt_mask(cpu)) == 2) {
>  		static_branch_inc_cpuslocked(&sched_smt_present);
> +#ifdef CONFIG_SCHED_CORE
> +		if (static_branch_unlikely(&__sched_core_enabled)) {
> +			rq->core_enabled = true;
> +		}
> +#endif
> +	}
> +
>  #endif
>  	set_cpu_active(cpu, true);
>  
> @@ -6447,8 +6518,16 @@ int sched_cpu_deactivate(unsigned int cpu)
>  	/*
>  	 * When going down, decrement the number of cores with SMT present.
>  	 */
> -	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
> +	if (cpumask_weight(cpu_smt_mask(cpu)) == 2) {
> +#ifdef CONFIG_SCHED_CORE
> +		struct rq *rq = cpu_rq(cpu);
> +		if (static_branch_unlikely(&__sched_core_enabled)) {
> +			rq->core_enabled = false;
> +		}
> +#endif
>  		static_branch_dec_cpuslocked(&sched_smt_present);
> +
> +	}
>  #endif
>  
>  	if (!sched_smp_initialized)

Aside from the fact that it's probably much saner to write this as:

	rq->core_enabled = static_key_enabled(&__sched_core_enabled);

I'm fairly sure I didn't write this part. And while I do somewhat see
the point of disabling core scheduling for a core that has only a single
thread on, I wonder why we care.

The thing is, this directly leads to the utter horror-show that is patch
6.

It should be perfectly possible to core schedule a core with only a
single thread on. It might be a tad silly to do, but it beats the heck
out of the trainwreck created here.

So how did this happen?