Re: [patch,v2] bdi: add a user-tunable cpu_list for the bdi flusher threads

[Jeff Moyer <jmoyer@redhat.com>]

Tejun Heo <tj@kernel.org> writes:

> Hmmm... cpu binding usually is done by kthread_bind() or explicit
> set_cpus_allowed_ptr() by the kthread itself.  The node part of the
> API was added later because there was no way to control where the
> stack is allocated and we often ended up with kthreads which are bound
> to a CPU with stack on a remote node.
>
> I don't know.  @node usually controls memory allocation and it could
> be surprising for it to control cpu binding, especially because most
> kthreads which are bound to CPU[s] require explicit affinity
> management as CPUs go up and down.  I don't know.  Maybe I'm just too
> used to the existing interface.

OK, I can understand this line of reasoning.

> As for the original patch, I think it's a bit too much to expose to
> userland.  It's probably a good idea to bind the flusher to the local
> node but do we really need to expose an interface to let userland
> control the affinity directly?  Do we actually have a use case at
> hand?

Yeah, folks pinning realtime processes to a particular cpu don't want
the flusher threads interfering with their latency.  I don't have any
performance numbers on hand to convince you of the benefit, though.

Cheers,
Jeff