On Fri, Aug 28, 2020 at 03:51:09PM -0400, Julien Desfossez wrote:
> + smt_weight = cpumask_weight(smt_mask);
> + for_each_cpu_wrap_or(i, smt_mask, cpumask_of(cpu), cpu) {
> + struct rq *rq_i = cpu_rq(i);
> + struct task_struct *p;
> +
> + /*
> + * During hotplug online a sibling can be added in
> + * the smt_mask * while we are here. If so, we would
> + * need to restart selection by resetting all over.
> + */
> + if (unlikely(smt_weight != cpumask_weight(smt_mask)))
> + goto retry_select;
cpumask_weigt() is fairly expensive, esp. for something that should
'never' happen.
What exactly is the race here?
We'll update the cpu_smt_mask() fairly early in secondary bringup, but
where does it become a problem?
The moment the new thread starts scheduling it'll block on the common
rq->lock and then it'll cycle task_seq and do a new pick.
So where do things go side-ways?
Can we please split out this hotplug 'fix' into a separate patch with a
coherent changelog.
