Re: [PATCHSET] workqueue: implement and use WQ_UNBOUND

[Tejun Heo <tj@kernel.org>]

Hello, David.

"David Howells" <dhowells@redhat.com> wrote:
> Does this mean you don't get reentrancy guarantees with unbounded work queues?

It means that unbound wq behaves like a generic worker pool.  Bound wq limits concurrency to minimal level but unbound one executes works as long as resources are available.  I'll continue below.

>I can't work out how you're achieving it with unbounded queues.  I presume with
>CPU-bound workqueues your doing it by binding the work item to the current CPU
>still...

Unbound works are served by a dedicated gcwq whose workers are not affine to any particular CPU.  As all unbound works are served by the same gcwq, non reentrancy is automatically guaranteed.

>Btw, how does this fare in an RT system, where work items bound to a CPU can't
>get executed because their CPU is busy with an RT thread, even though there are
>other, idle CPUs?

Sure, there's nothing special about unbound workers.  They're just normal kthreads.

>> Oh, and Frederic suggested that we would be better off with something based
>> on tracing API and I agree, so the debugfs thing is currently dropped from
>> the tree.  What do you think?
>
>I probably disagree.  I just want to be able to cat a file and see the current
>runqueue state.  I don't want to have to write and distribute a special program
>to do this.  Of course, I don't know that much about the tracing API, so
>cat'ing a file to get the runqueue listed nicely may be possible with that.

I'm relatively sure we can do that.  Frederic?

Thanks.

-- 
tejun