Date:	Thu, 20 Aug 2009 14:22:15 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org, jeff@...zik.org,
	benh@...nel.crashing.org, htejun@...il.com, bzolnier@...il.com,
	alan@...rguk.ukuu.org.uk,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 0/6] Lazy workqueues

On Thu, Aug 20, 2009 at 12:19:58PM +0200, Jens Axboe wrote:
> (sorry for the resend, but apparently the directory had some patches
>  in it already. plus, stupid git send-email doesn't default to
>  no chain replies, really annoying)
> 
> Hi,
> 
> After yesterdays rant on having too many kernel threads and checking
> how many I actually have running on this system (531!), I decided to 
> try and do something about it.
> 
> My goal was to retain the workqueue interface instead of coming up with
> a new scheme that required conversion (or converting to slow_work which,
> btw, is an awful name :-). I also wanted to retain the affinity
> guarantees of workqueues as much as possible.
> 
> So this is a first step in that direction, it's probably full of races
> and holes, but should get the idea across. It adds a
> create_lazy_workqueue() helper, similar to the other variants that we
> currently have. A lazy workqueue works like a normal workqueue, except
> that it only (by default) starts a core thread instead of threads for
> all online CPUs. When work is queued on a lazy workqueue for a CPU
> that doesn't have a thread running, it will be placed on the core CPUs
> list and that will then create and move the work to the right target.
> Should task creation fail, the queued work will be executed on the
> core CPU instead. Once a lazy workqueue thread has been idle for a
> certain amount of time, it will again exit.
> 
> The patch boots here and I exercised the rpciod workqueue and
> verified that it gets created, runs on the right CPU, and exits a while
> later. So core functionality should be there, even if it has holes.
> 
> With this patchset, I am now down to 280 kernel threads on one of my test
> boxes. Still too many, but it's a start and a net reduction of 251
> threads here, or 47%!
> 
> The code can also be pulled from:
> 
>   git://git.kernel.dk/linux-2.6-block.git workqueue
> 
> -- 
> Jens Axboe


That looks like a nice idea that may indeed solve the problem of thread
proliferation with per cpu workqueue.

Now I think there is another problem that taint the workqueues from the
beginning which is the deadlocks induced by one work that waits another
one in the same workqueue. And since the workqueues are executing the jobs
by serializing, the effect is deadlocks.

Often, drivers need to move from the central events/%d to a dedicated workqueue
because of that.

A idea to solve this:

We could have one thread per struct work_struct.
Similarly to this patchset, this thread waits for queuing requests, but only for
this work struct.
If the target cpu has no thread for this work, then create one, like you do, etc...

Then the idea is to have one workqueue per struct work_struct, which handles
per cpu task creation, etc... And this workqueue only handles the given work.

That may solve the deadlocks scenario that are often reported and lead to
dedicated workqueue creation.

That also makes disappearing the work execution serialization between different
worklets. We just keep the serialization between same work, which seems a
pretty natural thing and is less haphazard than multiple works of different
natures randomly serialized between them.

Note the effect would not only be a reducing of deadlocks but also probably
an increasing of throughput because works of different natures won't need anymore
to wait for the previous one completion.

Also a reducing of latency (a high prio work that waits for a lower prio
work).

There are good chances that we won't need any more per driver/subsys workqueue
creation after that, because everything would be per worklet.
We could use a single schedule_work() for all of them and not bother choosing
a specific workqueue or the central events/%d

Hmm?


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