Re: [PATCH v3 3/5] OPP: Improve require-opps linking

From: Saravana Kannan
Date: Thu Jul 25 2019 - 21:52:52 EST

On Wed, Jul 24, 2019 at 10:17 PM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> On 24-07-19, 21:09, Saravana Kannan wrote:
> > On Wed, Jul 24, 2019 at 8:07 PM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> > > We should be doing this whenever a new OPP table is created, and see
> > > if someone else was waiting for this OPP table to come alive.
> >
> > Searching the global OPP table list seems a ton more wasteful than
> > doing the lazy linking. I'd rather not do this.
> We can see how best to optimize that, but it will be done only once
> while a new OPP table is created and putting stress there is the right
> thing to do IMO. And doing anything like that in a place like
> opp-set-rate is the worst one. It will be a bad choice by design if
> you ask me and so I am very much against that.
> > > Also we
> > > must make sure that we do this linking only if the new OPP table has
> > > its own required-opps links fixed, otherwise delay further.
> >
> > This can be done. Although even without doing that, this patch is
> > making things better by not failing silently like it does today? Can I
> > do this later as a separate patch set series?
> I would like this to get fixed now in a proper way, there is no hurry
> for a quick fix currently. No band-aids please.
> > > Even then I don't want to add these checks to those places. For the
> > > opp-set-rate routine, add another flag to the OPP table which
> > > indicates if we are ready to do dvfs or not and mark it true only
> > > after the required-opps are all set.
> >
> > Honestly, this seems like extra memory and micro optimization without
> > any data to back it.
> Again, opp-set-rate isn't supposed to do something like this. It
> shouldn't handle initializations of things, that is broken design.
> > Show me data that checking all these table
> > pointers is noticeably slower than what I'm doing. What's the max
> > "required tables count" you've seen in upstream so far?
> Running anything extra (specially some initialization stuff) in
> opp-set-rate is wrong as per me and as a Maintainer of the OPP core it
> is my responsibility to not allow such things to happen.

Doing operations lazily right before they are needed isn't something
new in the kernel. It's done all over the place (VFP save/restore?).
It's not worth arguing though -- so I'll agree to disagree but follow
the Maintainer's preference.

> > I'd even argue that doing it the way I do might actually reduce the
> > cache misses/warm the cache because those pointers are going to be
> > searched/used right after anyway.
> So you want to make the cache hot with data, by running some code at a
> place where it is not required to be run really, and the fact that
> most of the data cached may not get used anyway ? And that is an
> improvement somehow ?

My point is that both of us are hypothesizing and for some
micro-optimization like this, data is needed.