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

[Tejun Heo <tj@kernel.org>]

Hello, Paul.

On Wed, Apr 26, 2023 at 02:55:04PM -0700, Paul E. McKenney wrote:
> But if the call_rcu_tasks_*() code detects too much lock contention on
> CPU 0's queue, which indicates that very large numbers of callbacks are
> being queued, it switches to per-CPU mode.  In which case, we are likely
> to have lots of callbacks on lots of queues, and in that case we really
> want to invoke them concurrently.
> 
> Then if a later grace period finds that there are no more callbacks, it
> switches back to CPU-0 mode.  So this extra workqueue overhead should
> happen only on systems with sparse cpu_online_masks that are under heavy
> call_rcu_tasks_*() load.

I still wonder whether it can be solved by simply switching to unbound
workqueues instead of implementing custom load-spreading mechanism. We'd be
basically asking the scheduler to what it thinks is best instead of trying
to make manual CPU placement decisions. That said, as a fix, the original
patch looks fine to me. Gonna go ack that.

Thanks.

-- 
tejun