linux-kernel - Re: [RFC][PATCH 1/4] sched: Force the address order of each sched class descriptor

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Message-ID: <20191220100033.GE2844@hirez.programming.kicks-ass.net>
Date:   Fri, 20 Dec 2019 11:00:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Kirill Tkhai <tkhai@...dex.ru>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH 1/4] sched: Force the address order of each sched
 class descriptor

On Fri, Dec 20, 2019 at 09:52:37AM +0100, Rasmus Villemoes wrote:
> On 19/12/2019 22.44, Steven Rostedt wrote:
> > From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> > 
> > In order to make a micro optimization in pick_next_task(), the order of the
> > sched class descriptor address must be in the same order as their priority
> > to each other. That is:
> > 
> >  &idle_sched_class < &fair_sched_class < &rt_sched_class <
> >  &dl_sched_class < &stop_sched_class
> > 
> > In order to guarantee this order of the sched class descriptors, add each
> > one into their own data section and force the order in the linker script.
> 
> I think it would make the code simpler if one reverses these, see other
> reply.

I started out agreeing, because of that mess around STOP_SCHED_CLASS and
that horrid BEFORE_CRUD thing.

Then, when I fixed it all up, I saw what it did to Kyrill's patch (#4)
and that ends up looking like:

-       if (likely((prev->sched_class == &idle_sched_class ||
-                   prev->sched_class == &fair_sched_class) &&
+       if (likely(prev->sched_class >= &fair_sched_class &&

And that's just weird.

Then I had a better look and now...

> > +/*
> > + * The order of the sched class addresses are important, as they are
> > + * used to determine the order of the priority of each sched class in
> > + * relation to each other.
> > + */
> > +#define SCHED_DATA				\
> > +	*(__idle_sched_class)			\
> > +	*(__fair_sched_class)			\
> > +	*(__rt_sched_class)			\
> > +	*(__dl_sched_class)			\
> > +	STOP_SCHED_CLASS

I'm confused, why does that STOP_SCHED_CLASS need magic here at all?
Doesn't the linker deal with empty sections already by making them 0
sized?

> >  /*
> >   * Align to a 32 byte boundary equal to the
> >   * alignment gcc 4.5 uses for a struct
> > @@ -308,6 +326,7 @@
> >  #define DATA_DATA							\
> >  	*(.xiptext)							\
> >  	*(DATA_MAIN)							\
> > +	SCHED_DATA							\
> >  	*(.ref.data)							\
> 
> Doesn't this make the structs end up in .data (writable) rather than
> .rodata?

Right! That wants fixing.