Re: [PATCH v6 15/17] static_call: Allow early init

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 13, 2020 at 04:24:19PM -0400, Steven Rostedt wrote:
> On Sat, 11 Jul 2020 07:08:31 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Fri, Jul 10, 2020 at 09:14:26PM -0400, Steven Rostedt wrote:
> > > On Fri, 10 Jul 2020 15:38:46 +0200
> > > Peter Zijlstra <peterz@infradead.org> wrote:
> > >   
> > > > In order to use static_call() to wire up x86_pmu, we need to
> > > > initialize earlier; copy some of the tricks from jump_label to enable
> > > > this.
> > > > 
> > > > Primarily we overload key->next to store a sites pointer when there
> > > > are no modules, this avoids having to use kmalloc() to initialize the
> > > > sites and allows us to run much earlier.
> > > >   
> > > 
> > > I'm confused. What was the need to have key->next store site pointers
> > > in order to move it up earlier?  
> > 
> > The critical part was to not need an allocation.
> 
> Why is an allocation needed? What's different about calling it early
> that we need an allocation or this trick?
> 
> The two paragraphs above seem totally disconnected.
> 
> "In order to use static_call() to wire up x86_pmu, we need to
> initialize earlier; copy some of the tricks from jump_label to enable
> this."
> 
> What tricks were copied?
> 
> "Primarily we overload key->next to store a sites pointer when there

^^ this trick...

+	union {
+		/* bit 0: 0 = mods, 1 = sites */
+		unsigned long type;
+		struct static_call_mod *mods;
+		struct static_call_site *sites;
+	};

If that isn't a trick, I don't know ;-)

> are no modules, this avoids having to use kmalloc() to initialize the
> sites and allows us to run much earlier."
> 
> Why is kmalloc() (or this trick) needed to initialize the sites?

That's just how the code was; it treated vmlinux as the NULL module, and
as such needed a static_call_mod allocated to host the static_call_sites
pointer.

That is, the static_call_key has a single linked list pointer to
static_call_mod, and every module has an entry on that list with a
pointer to their sites. Very simple and straight forward.

Except it requires an allocation to set up, which is a pain is you want
it initialized very early.