linux-kernel - Re: [PATCH 03/18] tty: Simplify tty buffer/ldisc interface with helper function

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Date:	Wed, 20 Mar 2013 13:49:56 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Ilya Zykov <linux@...k.ru>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, Min Zhang <mzhang@...sta.com>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/18] tty: Simplify tty buffer/ldisc interface with
 helper function

On Wed, 2013-03-20 at 16:47 +0400, Ilya Zykov wrote:
> >> I have little question about flush_to_ldisc().
> >> Does can it be multithreaded?
> >>
> >> I think yes, because on SMP schedule_work() can work on different CPU paralleled.
> > 
> > Yes, the same work item can now run in parallel on SMP since Tejun Heo
> > re-did the workqueue implementation on 2.6.36 [Stefan Richter, the
> > firewire maintainer, recently explained this history to me].
> 
> About multi threaded delayed works:
> 
> In many cases tty layer needs single threaded delayed work for each tty instance.
> I propose discussion about create for this purpose (tty layer)'s workqueue with WQ_NON_REENTRANT flag.
> And use it instead common schedule_work()'s workqueue - system_wq.
> I don't know how expensive(for system resource and CPU) it can be,
> but for tty layer, it can be very useful.

Ok. I agree it makes sense to explore the possible benefit.

There is another potential drawback to using non-reentrant workqueues.

The motivation for changing the workqueue api to allow parallel work
items on SMP was to fix a class of deadlocks where forward progress
could not be made due to subtle dependencies between work items
(actually that potential still exists with self-modifying work-items,
ie., work items that change their function).

The tty layer would need a detailed and thorough analysis of potential
dependencies to avoid creating problems. The drivers that use work items
might need examination as well.

Regards,
Peter Hurley


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