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

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Date:   Wed, 27 Jan 2021 12:27:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 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.

---
--- a/kernel/static_call.c
+++ b/kernel/static_call.c
@@ -8,6 +8,7 @@
 #include <linux/module.h>
 #include <linux/cpu.h>
 #include <linux/processor.h>
+#include <linux/kallsyms.h>
 #include <asm/sections.h>
 
 extern struct static_call_site __start_static_call_sites[],
@@ -325,8 +326,66 @@ static int __static_call_mod_text_reserv
 
 static int static_call_add_module(struct module *mod)
 {
-	return __static_call_init(mod, mod->static_call_sites,
-				  mod->static_call_sites + mod->num_static_call_sites);
+	struct static_call_site *start = mod->static_call_sites;
+	struct static_call_site *stop = start + mod->num_static_call_sites;
+	struct static_call_site *site;
+
+	struct {
+		unsigned long tramp;
+		unsigned long key;
+	} cache[8] = { { 0, 0}, };
+	int idx = 0;
+
+	for (site = start; site != stop; site++) {
+		unsigned long key, addr = (unsigned long)static_call_key(site);
+		unsigned long sym_size, sym_offset;
+		char sym_name[KSYM_NAME_LEN];
+		const char *name;
+		int i;
+
+		if (!kernel_text_address(addr))
+			continue;
+
+		/*
+		 * Gotta fix up the keys that point to the trampoline.
+		 */
+
+		/* Simple cache to avoid kallsyms */
+		for (i = 0; i < ARRAY_SIZE(cache); i++) {
+			if (cache[i].tramp == addr) {
+				key = cache[i].key;
+				goto got_key;
+			}
+		}
+
+		name = kallsyms_lookup(addr, &sym_size, &sym_offset, NULL, sym_name);
+		if (!name)
+			goto fail;
+
+		if (name != sym_name)
+			strcpy(sym_name, name);
+		memcpy(sym_name, STATIC_CALL_KEY_PREFIX_STR, STATIC_CALL_KEY_PREFIX_LEN);
+		key = kallsyms_lookup_name(sym_name);
+		if (!key)
+			goto fail;
+
+		/* Remember for next time.. */
+		cache[idx].tramp = addr;
+		cache[idx].key = key;
+		idx++;
+		idx %= ARRAY_SIZE(cache);
+
+got_key:
+		site->key = (key - (unsigned long)&site->key) |
+			    (site->key & STATIC_CALL_SITE_FLAGS);
+	}
+
+	return __static_call_init(mod, start, stop);
+
+fail:
+	pr_warn("Failed to fixup __raw_static_call() usage at: %ps\n",
+		static_call_addr(site));
+	return -EINVAL;
 }
 
 static void static_call_del_module(struct module *mod)