linux-kernel - Re: [PATCH 6/8] rbtree: Implement generic latch

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Date:	Fri, 20 Mar 2015 10:50:25 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mingo@...nel.org, rusty@...tcorp.com.au,
	mathieu.desnoyers@...icios.com, oleg@...hat.com,
	paulmck@...ux.vnet.ibm.com, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, andi@...stfloor.org,
	rostedt@...dmis.org, tglx@...utronix.de,
	Michel Lespinasse <walken@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Woodhouse <David.Woodhouse@...el.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 6/8] rbtree: Implement generic latch_tree

On Thu, Mar 19, 2015 at 11:04:33PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 19, 2015 at 01:58:33PM -0700, Andrew Morton wrote:
> > OK.  This code is basically required to support perf/ftrace and
> > modules, yes?  Presumably small and space-constrained systems aren't
> > using either, so they don't take the hit.
> > 
> > However CONFIG_MODULES systems which aren't using perf/ftrace _do_ take
> > a hit.  How many systems are we talking here?  All non-x86?
> 
> No, there's plenty !x86 systems that have NMI and perf/ftrace, in fact,
> ARM has them (FIQ) and there's event some ARM chips that use them for
> perf (i.MX6 is one).
> 
> But sure, we could make this depend on CONFIG_PERF_EVENTS ||
> CONFIG_TRACING.

A little something like so.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/module.c |   30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

--- a/kernel/module.c
+++ b/kernel/module.c
@@ -102,6 +102,8 @@ DEFINE_MUTEX(module_mutex);
 EXPORT_SYMBOL_GPL(module_mutex);
 static LIST_HEAD(modules);
 
+#if defined(CONFIG_PERF_EVENTS) || defined(CONFIG_TRACING)
+
 /*
  * Use a latched RB-tree for __module_address(); this allows us to use
  * RCU-sched lookups of the address from any context.
@@ -109,6 +111,10 @@ static LIST_HEAD(modules);
  * Because modules have two address ranges: init and core, we need two
  * latch_tree_nodes entries. We use the order they appear in struct module to
  * determine if we need to use the init or core values for the comparisons.
+ *
+ * This is conditional on PERF_EVENTS || TRACING because those can really hit
+ * __module_address() hard by doing a lot of stack unwinding; potentially from
+ * NMI context.
  */
 
 static __always_inline unsigned long __mod_tree_val(struct latch_tree_node *n)
@@ -185,7 +191,7 @@ static void mod_tree_remove(struct modul
 	mod_tree_remove_init(mod);
 }
 
-static struct module *mod_tree_find(unsigned long addr)
+static struct module *mod_find(unsigned long addr)
 {
 	struct latch_tree_node *ltn;
 
@@ -194,6 +200,26 @@ static struct module *mod_tree_find(unsi
 	return ltn ? ltn->priv : NULL;
 }
 
+#else /* !(PERF_EVENTS || TRACING) */
+
+static void mod_tree_insert(struct module *mod) { }
+static void mod_tree_remove_init(struct module *mod) { }
+static void mod_tree_remove(struct module *mod) { }
+
+static struct module *mod_find(unsigned long addr)
+{
+	struct module *mod;
+
+	list_for_each_entry_rcu(mod, &modules, list) {
+		if (within_module(addr, mod))
+			return mod;
+	}
+
+	return NULL;
+}
+
+#endif /* !(PERF_EVENTS || TRACING) */
+
 #ifdef CONFIG_KGDB_KDB
 struct list_head *kdb_modules = &modules; /* kdb needs the list of modules */
 #endif /* CONFIG_KGDB_KDB */
@@ -3939,7 +3965,7 @@ struct module *__module_address(unsigned
 
 	module_assert_mutex_or_preempt();
 
-	mod = mod_tree_find(addr);
+	mod = mod_find(addr);
 	if (mod) {
 		BUG_ON(!within_module(addr, mod));
 		if (mod->state == MODULE_STATE_UNFORMED)
--
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