linux-kernel - Re: [RFC PATCH 6/8] preempt/dynamic: Provide preempt_schedule[

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Date:   Wed, 27 Jan 2021 09:59:14 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Michal Hocko <mhocko@...e.com>,
        rostedt@...dmis.org, jbaron@...mai.com, ardb@...nel.org
Subject: Re: [RFC PATCH 6/8] preempt/dynamic: Provide
 preempt_schedule[_notrace]() static calls

On Wed, Jan 27, 2021 at 12:27:09PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 27, 2021 at 10:13:47AM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 26, 2021 at 05:57:30PM -0600, Josh Poimboeuf wrote:
> 
> > > Well, I hate it, but I'm not sure I have any better ideas.  It should be
> > > possible to use kallsyms, instead of the rb-tree/register nonsense.  Not
> > > sure about the performance impact though.  Might be a good reason to
> > > speed up kallsyms!
> > 
> > Oh right, let me see if I can make that work.
> 
> Something like so compiles.. but it does make the whole thing depend on
> KALLSYMS_ALL, which is somewhat yuck.
> 
> Also, kallsyms_lookup_name() is horrible, but not trivial to fix because
> of that compression scheme used.

The KALLSYMS_ALL dependency doesn't bother me personally but I assume
some of the tinyconfig folks might not appreciate it being on
permanently.

Can DEFINE_STATIC_CALL() make the tramp-key association?

e.g. have DEFINE_STATIC_CALL() add an entry to .static_call_tramp_key
which has an array of 

struct static_call_tramp_key {
	unsigned int tramp;  // PC-relative pointer to tramp
	unsigned int key;    // PC-relative pointer to key
}

and then just scan that instead of kallsyms.

-- 
Josh