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

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Date:	Wed, 09 Jun 2010 13:05:49 -0400
From:	James Bottomley <James.Bottomley@...e.de>
To:	Florian Mickler <florian@...kler.org>
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, 2010-06-09 at 18:32 +0200, Florian Mickler wrote:
> 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. 

The network latency notifier uses the value to recalculate something.
Losing the last value will mean it's using stale data.

> 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.

Well, the network notifier assumes the notifier value is the latest ... 

> 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. 

Actually, no ... the current API preserves ordering semantics.  If a
notifier sleeps and another change comes along, the update callsite will
sleep on the notifier's mutex ... so ordering is always preserved.

> 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. :)

The code I proposed does ... but it relies on the callsite supplying the
necessary context.

James


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