linux-kernel - Re: [PATCH v4] pm_qos: make update

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Message-ID: <20100609183204.1eeca494@schatten.dmk.lab>
Date:	Wed, 9 Jun 2010 18:32:04 +0200
From:	Florian Mickler <florian@...kler.org>
To:	James Bottomley <James.Bottomley@...e.de>
Cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	markgross@...gnar.org, mgross@...ux.intel.com,
	linville@...driver.com, Frederic Weisbecker <fweisbec@...il.com>,
	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] pm_qos: make update_request non blocking

On Wed, 09 Jun 2010 12:07:25 -0400
James Bottomley <James.Bottomley@...e.de> wrote:

> On Wed, 2010-06-09 at 18:00 +0200, Florian Mickler wrote:
> > On Wed, 09 Jun 2010 11:37:12 -0400
> > James Bottomley <James.Bottomley@...e.de> wrote:
> > 
> > 
> > > This still isn't resilient against two successive calls to update.  If
> > > the second one gets to schedule_work() before the work of the first one
> > > has finished, you'll corrupt the workqueue.
> > 
> > Sorry, I don't see it. Can you elaborate?
> > 
> > In "run_workqueue(" we do a list_del_init() which resets the
> > list-pointers of the workitem and only after that reset the
> > WORK_STRUCT_PENDING member of said structure. 
> > 
> > 
> > schedule_work does a queue_work_on which does a test_and_set_bit on
> > the WORK_STRUCT_PENDING member of the work and only queues the work
> > via list_add_tail in insert_work afterwards.
> > 
> > Where is my think'o? Or was this fixed while you didn't look?
> > 
> > So what _can_ happen, is that we miss a new notfication while the old
> > notification is still in the queue. But I don't think this is a problem.
> 
> OK, so the expression of the race is that the latest notification gets
> lost.  If something is tracking values, you'd really like to lose the
> previous one (which is now irrelevant) not the latest one.  The point is
> there's still a race.
> 
> James
> 

Yeah, but for blocking notification it is not that bad. 

Doesn't using blocking notifiers mean that you need to always check
via pm_qos_request() to get the latest value anyways? I.e. the
notification is just a hint, that something changed so you can act on
it. But if you act (may it because of notification or because of
something other) then you have to get the current value anyways.

I think there are 2 schemes of operation. The one which needs
atomic notifiers and where it would be bad if we lost any notification
and the other where it is just a way of doing some work in a timely
fashion but it isn't critical that it is done right away.

pm_qos was the second kind of operation up until now, because the
notifiers may have got scheduled away while blocked. 

I think we should allow for both kinds of operations simultaneous. But
as I got that pushback to the change from John Linville as I touched his
code, I got a bit reluctant and just have done the simple variant. :)

(for example if we want to implement some sort of debugging stats, we
could use blocking notifications, but if we for example block some
c-stats due to a latency-requirement we probably need some atomic
notifications.)

Cheers,
Flo
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