Re: [patch 20/38] Minor fault path optimization.

[Christoph Hellwig <hch@infradead.org>]

On Sat, Apr 28, 2007 at 02:49:55PM +1000, Paul Mackerras wrote:
> Martin Schwidefsky writes:
> 
> > The minor fault path has grown a lot in terms of cycles. In particular
> > the kprobes hook is very costly. Optimize the path to save a couple of
> > cycles. If kprobes is enabled more than 300 cycles can be avoided if 
> > kprobes_running() is false.
> 
> There's no good reason to use a notifier for page faults, since
> there's only one external piece of code that wants to know about
> them...

Well, here's my powerpc patch to do the direct call that I sent out
a few weeks ago.  It not just speed up the pagefault path a lot, 
but also is a major code cleanup.  Andi and Anton didn't like it
because they have ambition for murky debugger code to possibly
use it one day..

Index: linux-2.6.20/arch/powerpc/mm/fault.c
===================================================================
--- linux-2.6.20.orig/arch/powerpc/mm/fault.c	2007-03-19 14:50:41.000000000 +0100
+++ linux-2.6.20/arch/powerpc/mm/fault.c	2007-03-19 15:00:00.000000000 +0100
@@ -39,37 +39,26 @@
 #include <asm/kdebug.h>
 #include <asm/siginfo.h>
 
-#ifdef CONFIG_KPROBES
-ATOMIC_NOTIFIER_HEAD(notify_page_fault_chain);
 
-/* Hook to register for page fault notifications */
-int register_page_fault_notifier(struct notifier_block *nb)
+#ifdef CONFIG_KPROBES
+static inline int notify_page_fault(struct pt_regs *regs)
 {
-	return atomic_notifier_chain_register(&notify_page_fault_chain, nb);
-}
+	int ret = 0;
 
-int unregister_page_fault_notifier(struct notifier_block *nb)
-{
-	return atomic_notifier_chain_unregister(&notify_page_fault_chain, nb);
-}
+	/* kprobe_running() needs smp_processor_id() */
+	if (!user_mode(regs)) {
+		preempt_disable();
+		if (kprobe_running() && kprobe_fault_handler(regs, 11))
+			ret = 1;
+		preempt_enable();
+	}
 
-static inline int notify_page_fault(enum die_val val, const char *str,
-			struct pt_regs *regs, long err, int trap, int sig)
-{
-	struct die_args args = {
-		.regs = regs,
-		.str = str,
-		.err = err,
-		.trapnr = trap,
-		.signr = sig
-	};
-	return atomic_notifier_call_chain(&notify_page_fault_chain, val, &args);
+	return ret;
 }
 #else
-static inline int notify_page_fault(enum die_val val, const char *str,
-			struct pt_regs *regs, long err, int trap, int sig)
+static inline int notify_page_fault(struct pt_regs *regs)
 {
-	return NOTIFY_DONE;
+	return 0;
 }
 #endif
 
@@ -175,8 +164,7 @@ int __kprobes do_page_fault(struct pt_re
 	is_write = error_code & ESR_DST;
 #endif /* CONFIG_4xx || CONFIG_BOOKE */
 
-	if (notify_page_fault(DIE_PAGE_FAULT, "page_fault", regs, error_code,
-				11, SIGSEGV) == NOTIFY_STOP)
+	if (notify_page_fault(regs))
 		return 0;
 
 	if (trap == 0x300) {
Index: linux-2.6.20/arch/powerpc/kernel/kprobes.c
===================================================================
--- linux-2.6.20.orig/arch/powerpc/kernel/kprobes.c	2007-03-19 14:53:01.000000000 +0100
+++ linux-2.6.20/arch/powerpc/kernel/kprobes.c	2007-03-19 15:03:41.000000000 +0100
@@ -376,7 +376,7 @@ out:
 	return 1;
 }
 
-static int __kprobes kprobe_fault_handler(struct pt_regs *regs, int trapnr)
+int __kprobes kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 {
 	struct kprobe *cur = kprobe_running();
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
@@ -461,14 +461,6 @@ int __kprobes kprobe_exceptions_notify(s
 		if (post_kprobe_handler(args->regs))
 			ret = NOTIFY_STOP;
 		break;
-	case DIE_PAGE_FAULT:
-		/* kprobe_running() needs smp_processor_id() */
-		preempt_disable();
-		if (kprobe_running() &&
-		    kprobe_fault_handler(args->regs, args->trapnr))
-			ret = NOTIFY_STOP;
-		preempt_enable();
-		break;
 	default:
 		break;
 	}
Index: linux-2.6.20/include/asm-powerpc/kdebug.h
===================================================================
--- linux-2.6.20.orig/include/asm-powerpc/kdebug.h	2007-03-19 14:55:13.000000000 +0100
+++ linux-2.6.20/include/asm-powerpc/kdebug.h	2007-03-19 15:03:27.000000000 +0100
@@ -18,8 +18,20 @@ struct die_args {
 
 extern int register_die_notifier(struct notifier_block *);
 extern int unregister_die_notifier(struct notifier_block *);
-extern int register_page_fault_notifier(struct notifier_block *);
-extern int unregister_page_fault_notifier(struct notifier_block *);
+
+/*
+ * These are only here because kprobes.c wants them to implement a
+ * blatant layer violation.  Will hopefully go away soon once all
+ * architectures are updated.
+ */
+static inline int register_page_fault_notifier(struct notifier_block *nb)
+{
+	return 0;
+}
+static inline int unregister_page_fault_notifier(struct notifier_block *nb)
+{
+	return 0;
+}
 extern struct atomic_notifier_head powerpc_die_chain;
 
 /* Grossly misnamed. */
@@ -29,7 +41,6 @@ enum die_val {
 	DIE_DABR_MATCH,
 	DIE_BPT,
 	DIE_SSTEP,
-	DIE_PAGE_FAULT,
 };
 
 static inline int notify_die(enum die_val val,char *str,struct pt_regs *regs,long err,int trap, int sig)
Index: linux-2.6.20/include/asm-powerpc/kprobes.h
===================================================================
--- linux-2.6.20.orig/include/asm-powerpc/kprobes.h	2007-03-19 15:01:14.000000000 +0100
+++ linux-2.6.20/include/asm-powerpc/kprobes.h	2007-03-19 15:01:55.000000000 +0100
@@ -100,5 +100,6 @@ struct kprobe_ctlblk {
 
 extern int kprobe_exceptions_notify(struct notifier_block *self,
 					unsigned long val, void *data);
+extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
 #endif /* __KERNEL__ */
 #endif	/* _ASM_POWERPC_KPROBES_H */