Re: [PATCH 00/10] workqueue: restructure flush_workqueue() and start all flusher at the same time

[Tejun Heo <tj@kernel.org>]

Hello, Lai.

On Tue, Sep 25, 2012 at 05:02:43PM +0800, Lai Jiangshan wrote:
> It is not possible to remove cascading. If cascading code is
> not in flush_workqueue(), it must be in some where else.

Yeah, sure, I liked that it didn't have to be done explicitly as a
separate step.

> If you force overflow to wait for freed color before do flush(which also
> force only one flusher for one color), and force the sole flush_workqueue()
> to grab ->flush_mutex twice, we can simplify the flush_workqueue().
> (see the attached patch, it remove 100 LOC, and the cascading code becomes
> only 3 LOC). But these two forcing slow down the caller a little.

Hmmm... so, that's a lot simpler.  flush_workqueue() isn't a super-hot
code path and I don't think grabbing mutex twice is too big a deal.  I
haven't actually reviewed the code but if it can be much simpler and
thus easier to understand and verify, I might go for that.

> (And if you allow to use SRCU(which is only TWO colors), you can remove another
> 150 LOC. flush_workqueue() will become single line. But it will add some more overhead
> in flush_workqueue() because SRCU's readsite is lockless)

I'm not really following how SRCU would factor into this but
supporting multiple colors was something explicitly requested by
Linus.  The initial implementation was a lot simpler which supported
only two colors.  Linus was worried that the high possibility of
flusher clustering could lead to chaining of latencies.

Thanks.

-- 
tejun