linux-kernel - Re: [PATCH] firmware_class: Move request_firmware

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Message-Id: <201203162245.52281.rjw@sisk.pl>
Date:	Fri, 16 Mar 2012 22:45:52 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Christian Lamparter <chunkeey@...glemail.com>,
	linux-kernel@...r.kernel.org,
	Linux PM mailing list <linux-pm@...r.kernel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	alan@...rguk.ukuu.org.uk,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Saravana Kannan <skannan@...eaurora.org>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: [PATCH] firmware_class: Move request_firmware_nowait() to workqueues

On Friday, March 16, 2012, Stephen Boyd wrote:
> On 03/16/12 13:19, Rafael J. Wysocki wrote:
> > On Friday, March 16, 2012, Stephen Boyd wrote:
> >> On 03/15/12 15:31, Rafael J. Wysocki wrote:
> >>> On Thursday, March 15, 2012, Stephen Boyd wrote:
> >>>> On 03/15/12 13:07, Christian Lamparter wrote:
> >>>>> On Thursday, March 15, 2012 08:50:15 PM Stephen Boyd wrote:
> >>>>>> Oddly enough a work_struct was already part of the firmware_work
> >>>>>> structure but nobody was using it. Instead of creating a new
> >>>>>> kthread for each request_firmware_nowait() just schedule the work
> >>>>>> on the system workqueue. This should avoid some overhead in
> >>>>>> forking new threads when they're not strictly necessary if
> >>>>>> workqueues are available.
> >>>>>>
> >>>>>> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> >>>>>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> >>>>>> Cc: Kay Sievers <kay.sievers@...y.org>
> >>>>>> Cc: Rafael J. Wysocki <rjw@...k.pl>
> >>>>>> ---
> >>>>>>
> >>>>>> I saw this while looking at this problem we're having.
> >>>>> Correct me if I'm wrong, but wouldn't that stall all other
> >>>>> global workqueue tasks for up to 60 seconds [in worst case]?
> >>>>>
> >>>>> But I think we can get rid of the firmware_work work struct...
> >>>>>
> >>>> My understanding is that with concurrency managed workqueues when the
> >>>> work item blocks another will be scheduled to run almost immediately. So
> >>>> before that change by Tejun workqueues would have been a bad idea
> >>>> because it could have blocked up to 60 second but now it should be fine
> >>>> because that work item will just be put to sleep and another request
> >>>> will run.
> >>> Please read the description of system_wq in workqueue.h.
> >>>
> >>> You should have used either system_long_wq or system_nrt_wq (depending on
> >>> what you really need).
> >>>
> >>>
> >> Thanks. I think we can use system_nrt_wq then? Or maybe even the
> >> unbounded workqueue system_unbound_wq?
> > Hmm.  Can you please remind me what the exact role of that work item is?
> >
> > It loads the device's firmware, but I'm not sure in what situations that's
> > supposed to happen.
> >
> 
> request_firmware_nowait() is used by code that wants to get the firmware
> asynchronously. Callers pass in a callback function which is called once
> the firmware is retrieved. The work item will correspond to one call to
> request_firmware_nowait(), where the work item will handle the sysfs
> entry generation, uevent generation, and wait_for_completion() calls
> that _request_firmware() does.
> 
> The work item also executes the callback function the caller passes in
> which could do probably anything and could take an arbitrarily long
> time. It looks like some drivers even chain request_firmware_nowait()
> together by calling request_firmware_nowait() from the callback functions.

So it looks like an unbound workqueue would be suitable for that, but
perhaps it may even be an ordered one?

Rafael
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