linux-kernel - Re: [Ksummit-2008-discuss] Delayed interrupt work, thread pools

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Message-ID: <Pine.LNX.4.64.0807021524020.2151@blonde.site>
Date:	Wed, 2 Jul 2008 15:27:09 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	Dean Nelson <dcn@....com>,
	ksummit-2008-discuss@...ts.linux-foundation.org,
	Robin Holt <holt@....com>,
	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [Ksummit-2008-discuss] Delayed interrupt work, thread pools

On Wed, 2 Jul 2008, Benjamin Herrenschmidt wrote:
> 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.

Do the pdflush daemons (from mm/pdflush.c) provide another example?

Hugh
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