Subject: Re: Delayed interrupt work, thread pools
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reply-To: benh@kernel.crashing.org
To: Dean Nelson <dcn@sgi.com>
Cc: Robin Holt <holt@sgi.com>, ksummit-2008-discuss@lists.linux-foundation.org,
       Linux Kernel list <linux-kernel@vger.kernel.org>
In-Reply-To: <20080702013927.GA2264@sgi.com>
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	 <20080701130240.GD10511@sgi.com>  <20080702013927.GA2264@sgi.com>
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Date: Wed, 02 Jul 2008 12:38:52 +1000
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On Tue, 2008-07-01 at 20:39 -0500, Dean Nelson wrote:
> As Robin, mentioned XPC manages a pool of kthreads that can (for performance
> reasons) be quickly awakened by an interrupt handler and that are able to
> block for indefinite periods of time.
> 
> In drivers/misc/sgi-xp/xpc_main.c you'll find a rather simplistic attempt
> at maintaining this pool of kthreads. 
> 
> The kthreads are activated by calling xpc_activate_kthreads(). Either idle
> kthreads are awakened or new kthreads are created if a sufficent number of
> idle kthreads are not available.
> 
> Once finished with current 'work' a kthread waits for new work by calling
> wait_event_interruptible_exclusive(). (The call is found in
> xpc_kthread_waitmsgs().)
> 
> The number of idle kthreads is limited as is the total number of kthreads
> allowed to exist concurrently.
> 
> It's certainly not optimal in the way it maintains the number of kthreads
> in the pool over time, but I've not had the time to spare to make it better.
> 
> I'd love it if a general mechanism were provided so that XPC could get out
> of maintaining its own pool.

Thanks. That makes one existing in-tree user and a one likely WIP user,
probably enough to move forward :-)

I'll look at your implementation and discuss internally see what our
specific needs in term of number of threads etc... look like.

I might come up with something simple first (ie, generalizing your
current implementation for example) and then look at some smarter
management of the thread pools.

Cheers,
Ben.


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