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

Peter Zijlstra
2020-04-14T11:37:24Z

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?