linux-kernel - Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Message-Id: <201006032302.59947.rjw@sisk.pl>
Date:	Thu, 3 Jun 2010 23:02:59 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	markgross@...gnar.org
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	Florian Mickler <florian@...kler.org>, 640e9920@...il.com,
	Alan Stern <stern@...land.harvard.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Brian Swetland <swetland@...gle.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Thursday 03 June 2010, mark gross wrote:
> On Thu, Jun 03, 2010 at 12:03:49AM +0200, Rafael J. Wysocki wrote:
> > On Wednesday 02 June 2010, mark gross wrote:
> > > On Tue, Jun 01, 2010 at 08:50:02PM -0700, Arve Hjønnevåg wrote:
> > > > On Tue, Jun 1, 2010 at 7:05 AM, mark gross <640e9920@...il.com> wrote:
> > > > > On Tue, Jun 01, 2010 at 09:07:37AM +0200, Florian Mickler wrote:
> > > > ...
> > > > >> +static void update_target_val(int pm_qos_class, s32 val)
> > > > >> +{
> > > > >> +     s32 extreme_value;
> > > > >> +     s32 new_value;
> > > > >> +     extreme_value = atomic_read(&pm_qos_array[pm_qos_class]->target_value);
> > > > >> +     new_value = pm_qos_array[pm_qos_class]->comparitor(val,extreme_value);
> > > > >> +     if (extreme_value != new_value)
> > > > >> +             atomic_set(&pm_qos_array[pm_qos_class]->target_value,new_value);
> > > > >> +}
> > > > >> +
> > > > >
> > > > > Only works 1/2 the time, but I like the idea!
> > > > > It fails to get the righ answer when constraints are reduced.  But, this
> > > > > idea is a good improvement i'll roll into the next pm_qos update!
> > > > >
> > > > 
> > > > I think it would be a better idea to track your constraints with a
> > > > sorted data structure. That way you can to better than O(n) for both
> > > > directions. If you have a lot of constraints with the same value, it
> > > > may even be worthwhile to have a two stage structure where for
> > > > instance you use a rbtree for the unique values and list for identical
> > > > constraints.
> > > 
> > > I don't agree, we went through this tree vrs list discussion a few times
> > > before in other areas of the kernel.  Wherever the list tended to be
> > > short, a simple list wins.  However; we can try it, after we have some
> > > metrics and stress test cases identified we can measure its effectivenes
> > > against.
> > 
> > How many different values are there to handle?
> >
> 
> for the current pm_qos users its tiny.  I've never heard of more than a
> few < 10.  However; for the new "interactive" class to provide suspend
> blocker functionality, I expect the number to be up to 20.
> 
> but realistically I bet we never get more than 10ish.
> 
> One constraint constraint request per module from isr to user mode.
> Once in user mode there would be only a few (assuming Android user
> space) I think just from the power HAL, input HAL, and the RIL.
> 
> Still a pretty small number I don't think we need to worry about scaling
> as much as we need to worry about performance.

In that case sorting the structure is rather not going to improve things, but
it might be worth using a hashtable or something similar.

Rafael
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