Message-ID: <20100605040638.GB20419@gvim.org>
Date:	Fri, 4 Jun 2010 21:06:38 -0700
From:	mark gross <640e9920@...il.com>
To:	James Bottomley <James.Bottomley@...e.de>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, markgross@...gnar.org,
	640e9920@...il.com, Peter Zijlstra <peterz@...radead.org>,
	Brian Swetland <swetland@...gle.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: PM_QOS re-design for hot path use of limited dynamic range
 requests was- Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Fri, Jun 04, 2010 at 09:23:10AM -0500, James Bottomley wrote:
> On Thu, 2010-06-03 at 21:07 -0700, Arve Hjønnevåg wrote:
> > On Thu, Jun 3, 2010 at 2:05 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > > On Thursday 03 June 2010, James Bottomley wrote:
> > >> On Thu, 2010-06-03 at 00:10 -0700, Arve Hjønnevåg wrote:
> > >> > On Wed, Jun 2, 2010 at 10:40 PM, mark gross <640e9920@...il.com> wrote:
> > >> > > On Wed, Jun 02, 2010 at 09:54:15PM -0700, Brian Swetland wrote:
> > >> > >> On Wed, Jun 2, 2010 at 8:18 PM, mark gross <640e9920@...il.com> wrote:
> > >> > >> > On Wed, Jun 02, 2010 at 02:58:30PM -0700, Arve Hjønnevåg wrote:
> > >> > >> >>
> > >> > >> >> The list is not short. You have all the inactive and active
> > >> > >> >> constraints on the same list. If you change it to a two level list
> > >> > >> >> though, the list of unique values (which is the list you have to walk)
> > >> > >> >> may be short enough for a tree to be overkill.
> > >> > >> >
> > >> > >> > what have you seen in practice from the wake-lock stats?
> > >> > >> >
> > >> > >> > I'm having a hard time seeing where you could get more than just a
> > >> > >> > handfull.  However; one could go to a dual list (like the scheduler) and
> > >> > >> > move inactive nodes from an active to inactive list, or we could simply
> > >> > >> > remove them from the list uppon inactivity.  which would would well
> > >> > >> > after I change the api to have the client allocate the memory for the
> > >> > >> > nodes...  BUT, if your moving things in and out of a list a lot, I'm not
> > >> > >> > sure the break even point where changing the structure helps.
> > >> > >> >
> > >> > >> > We'll need to try it.
> > >> > >> >
> > >> > >> > I think we will almost never see more than 10 list elements.
> > >> > >> >
> > >> > >> > --mgross
> > >> > >> >
> > >> > >> >
> > >> > >>
> > >> > >> I see about 80 (based on the batteryinfo dump) on my Nexus One
> > >> > >> (QSD8250, Android Froyo):
> > >> > >
> > >> > > shucks.
> > >> > >
> > >> > > well I think for a pm_qos class that has boolean dynamic range we can
> > >> > > get away with not walking the list on every request update.  we can use
> > >> > > a counter, and the list will be for mostly for stats.
> > >> > >
> > >> >
> > >> > Did you give any thought to my suggestion to only use one entry per
> > >> > unique value on the first level list and then use secondary lists of
> > >> > identical values. That way if you only have two constraints values the
> > >> > list you have to walk when updating a request will never have more
> > >> > than two entries regardless of how many total request you have.
> > >> >
> > >> > A request update then becomes something like this:
> > >> >   if on primary list {
> > >> >     unlink from primary list
> > >> >     if secondary list is not empty
> > >> >       get next secondary entry and add in same spot on primary list
> > >> >   }
> > >> >   unlink from secondary list
> > >> >   find new spot on primary list
> > >> >   if already there
> > >> >     add to secondary list
> > >> >   else
> > >> >     add to primary list
> > >>
> > >> This is just reinventing hash bucketed lists.  To get the benefits, all
> > >> we do is implement an N state constraint as backed by an N bucketed hash
> > >> list, which the kernel already has all the internal mechanics for.
> > >
> > > Agreed.
> > >
> > 
> > No, a hash is used for quick lookup of a specific value, not to find
> > an extreme value.
> 
> If you only have N possible values an N bucket hash list is rather
> efficient (provided N is small).  But I would agree that knowing what N
> is represents an API change, and since plists can do this without
> changing the API, they're better.
> 
> >  It is however extremely similar to plists. The only
> > difference is that plists link all the secondary lists together.
> 
> Right, so they would solve the *current* problem exactly.
> 
> >  If we
> > want to have constraints that autoexpire, then keeping the secondary
> > lists separate allows the same optimization as I did for
> > wakelock/suspend_blocker timeouts where no timer is active if an
> > (equal or stricter) non-expiring constraint is active.
> 
> But this is a future discussion and not part of the patch.  The way open
> source works is that we sort out the best implementation for the current
> conditions.  If the implementation has to change because of future
> stuff, then we change it when the future stuff comes along.  Changing
> implementations is easy (they don't have any externally visible impact).
> Changing the in-kernel API is slightly harder, but easily doable.  It's
> only changing the user visible ABI that we worry about and try not to
> do.
>

True.

The following is what I think I'll work on this weekend.  (hopefully
have some sort of patch that at least compiles...)


Changes to pm_qos to enable "wakelock" or "suspend blocker" support:

Requirements:
1) atomic context support.
2) make updating request fast enough for hot path users.
3) Add a request class for "interactive_suspend", this particular
request has a dynamic range of 1 (its 0 or 1).  If zero then ok to auto
suspend, if 1 then only user driven suspend.  Whenever the aggregate
request changes value the registered notifiers are called.


Implementation:
* change api to have caller allocated the qos request structures. (solve
the kalloc issue)
* add plist use for constraint classes with hash-able dynamic ranges
* consider auto hashing / plist-ing everything.  (most users of pm_qos
don't do any fancy stuff and tend to request the only 2 different
values, ever)  but for now only request with dynamic range of 0,1 will
get the fancy lists.

I think the rest are implementation details.

--mgross



 
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