Re: [PATCH RFC rcu] Stop rcu_tasks_invoke_cbs() from using never-online CPUs

[Tejun Heo]

Hello,

On Wed, Apr 26, 2023 at 02:17:03PM -0700, Paul E. McKenney wrote:
> But the idea here is to spread the load of queueing the work as well as
> spreading the load of invoking the callbacks.
> 
> I suppose that I could allocate an array of ints, gather the online CPUs
> into that array, and do a power-of-two distribution across that array.
> But RCU Tasks allows CPUs to go offline with queued callbacks, so this
> array would also need to include those CPUs as well as the ones that
> are online.

Ah, I see, so it needs to make the distinction between cpus which have never
been online and are currently offline but used to be online.

> Given that the common-case system has a dense cpus_online_mask, I opted
> to keep it simple, which is optimal in the common case.
> 
> Or am I missing a trick here?

The worry is that on systems with actual CPU hotplugging, cpu_online_mask
can be pretty sparse - e.g. 1/4 filled wouldn't be too out there. In such
cases, the current code would end scheduling the work items on the issuing
CPU (which is what WORK_CPU_UNBOUND does) 3/4 of the time which probably
isn't the desired behavior.

So, I can initialize all per-cpu workqueues for all possible cpus on boot so
that rcu doesn't have to worry about it but that would still have a similar
problem of the callbacks not really being spread as intended.

I think it depends on how important it is to spread the callback workload
evenly. If that matters quite a bit, it probably would make sense to
maintain a cpumask for has-ever-been-online CPUs. Otherwise, do you think it
can just use an unbound workqueue and forget about manually distributing the
workload?

Thanks.

-- 
tejun