Hello,
On Sat, May 17, 2014 at 03:41:55PM +0200, Frederic Weisbecker wrote:
> > > - last_pool = get_work_pool(work);
> > > + last_pool = wq->flags & __WQ_ORDERED ? NULL : get_work_pool(work);
> > > if (last_pool && last_pool != pwq->pool) {
> > > struct worker *worker;
> >
> > I'm not a big fan of the fact that ordered queues need to be handled
> > differently when queueing, but as the code is currently written, this
> > is pretty much necessary to maintain execution order, right?
> >
> > Otherwise, you end up with requeueing work items targeting the pwq it
> > was executing on and new ones targeting the newest one screwing up the
> > ordering. I think that'd be a lot more important to note in the
> > comment. This is a correctness measure. Back-to-back requeueing
> > being affected by this is just a side-effect.
>
> In the case of ordered workqueues it actually doesn't matter much in
> term of ordering. But it's needed when pwqs are replaced (as it happens
> in apply_workqueue_attrs(). We must make sure works requeueing themselves
> don't always requeue to the old pwq otherwise it will never be able to
> switch and be released. Also the next work items will be queued on the next
But that's the same for other pwqs too. Back-to-back requeueing will
hold back pwq switching on any workqueue.
> pwq but this one will never be able to run due to the old workqueue still
> being used by the item requeing itself. So we also risk starvation on the
> new workqueue.
>
> But the ordering itself is actually fine for ordered workqueue. It's actually
> enforced by the fact that only one pwq can run at a time for a given
> workqueue.
Maybe I'm confused but I don't think it'd be. Let's say there was an
attribute change with one work item, A, which is performing
back-to-back requeueing and another one, B, which queues itself
intermittently. If B is queued while A is executing, followed by A
requeueing itself, the expected execution order is A - B - A; however,
without the above exception for ordered workqueues, it'd end up A - A
- B because B will end up on the new pwq while A on the older one and
max_active won't be transferred to the new pwq before it becomes
empty.
> > Just collapse it into the calling function. This obfuscates more than
> > helps.
>
> Yeah but the condition is already big. Lets hope the result won't be too ugly.
I didn't mean that the condition should be encoded in the if
conditional. It's fine to break it out using a separate variable or
whatever. I just don't think breaking it out to a separate function
is helping anything.
Thanks.
--
tejun
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