Re: [PATCH 0/3] Convert libata pio task to slow-work

[Jens Axboe]

On Thu, Aug 27 2009, Tejun Heo wrote:
> Hello, Jens.
> 
> Jens Axboe wrote:
> > Hi,
> > 
> > This patchset adds support for slow-work for delayed slow work and
> > for cancelling slow work. Note that these patches are totally
> > untested!
> 
> As what I'm currently working on is likely to collide with these
> changes, here is a short summary of what's been going on.
> 
> /* excerpted from internal weekly work report and edited */
> 
> The principle is the same as I described before.  It hooks into the
> scheduler using an alias scheduler class of sched_fair and gets
> notifications of workqueue threads going into sleep, waking up and
> getting preempted from which worker pool is managed automatically for
> full concurrency with the least number of concurrent threads.
> 
> There's a global workqueue per-cpu and each actual workqueue is front
> to the global one adding necessary attributes and/or defining a
> flushing domain.  Each global workqueue can have multiple workers
> (upto 128 in the current incarnation) and creates and kicks new ones
> as necessary to keep the cpu occupied.
> 
> The diffcult part was teaching workqueue how to handle multiple
> workers yet maintaining its exclusion properties, flushing rules and
> forward progress guarantees - a single work can't be running
> concurrently on the same cpu but can across different cpus,
> flush_work() deals with single cpu flushing but others deal with all
> the cpus and so on.  Because each work struct can't be accessed once
> the work actually begins running, keeping track of things become
> somewhat difficult as multiple workers now process works from a single
> queue.  Anyways, after much head scratching, I think most problems
> have been nailed down although I wouldn't know for sure till I get it
> actually working.
> 
> There's slight more book keeping to do on each work-processing
> iteration but overall I think it will be a win considering that it can
> remove unnecessary task switchings, usage of different stacks (cache
> foot-print) and cross-cpu work bouncing (for currently single threaded
> workqueues).  If it really works as expected, it should be able to
> replace async, [very]_slow_work and remove most of private workqueues
> while losing no concurrency or forward-progress guarantees, which
> would be pretty decent.
> 
> /**/
> 
> I finished first draft implementation and review pass yesterday and it
> seems like there shouldn't be any major problem now but I haven't even
> tried to compile it yet, so I'm not yet entirely sure how it would
> eventually turn out and if I hit some major roadblock I might just
> drop it.
> 
> It would be nice if merging of this series and the lazy work can be
> held a bit but there's no harm in merging either.  If the concurrency
> managed workqueue turns out to be a good idea, we can replace it then.

It can wait, what you describe above sounds really cool and would
hopefully allow us to get rid of all workqueues (provided it scales well
and doesn't fall down on cache line contention with many different
instances pounding on it).

Care to post it? I know you don't think it's perfect yet, but it would
make a lot more sense to throw effort into this rather than waste time
on partial solutions.

-- 
Jens Axboe

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