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

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Date:   Wed, 27 Jan 2021 10:33:08 -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 05:19:02PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 27, 2021 at 09:59:14AM -0600, Josh Poimboeuf wrote:
> > 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.
> 
> I suppose this ought to cure the _ALL thing.
> 
> diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
> index 7ecd2ccba531..83586cc4d954 100644
> --- a/scripts/kallsyms.c
> +++ b/scripts/kallsyms.c
> @@ -260,6 +260,12 @@ static int symbol_valid(const struct sym_entry *s)
>  {
>  	const char *name = sym_name(s);
>  
> +	/*
> +	 * Always emit __SCK__ symbols for static_call_add_module().
> +	 */
> +	if (!strncmp(name, "__SCK__", 7))
> +		return 1;
> +
>  	/* if --all-symbols is not specified, then symbols outside the text
>  	 * and inittext sections are discarded */
>  	if (!all_symbols) {

This made me LOL for some reason.  I like it though.

What did you think about .static_call_tramp_key?  I could whip up a
patch later unless you beat me to it.

-- 
Josh