Re: [PATCH] Only process_die notifier in ia64_do_page_fault if KPROBES is configured.

Rusty Lynch
2005-08-30T00:19:23Z

On Sat, Aug 27, 2005 at 02:24:25AM +0200, Andi Kleen wrote:
> On Saturday 27 August 2005 01:05, Christoph Lameter wrote:
> > On Fri, 26 Aug 2005, Rusty Lynch wrote:
> > > Just to be sure everyone understands the overhead involved, kprobes only
> > > registers a single notifier. If kprobes is disabled (CONFIG_KPROBES is
> > > off) then the overhead on a page fault is the overhead to execute an
> > > empty notifier chain.
> >
> > Its the overhead of using registers to pass parameters, performing a
> > function call that does nothing etc. A waste of computing resources. All
> > of that unconditionally in a performance critical execution path that
> > is executed a gazillion times for an optional feature that I frankly
> > find not useful at all and that is disabled by default.
>
> In the old days notifier_call_chain used to be inline. Then someone looking
> at code size out of lined it. Perhaps it should be inlined again or notifier.h
> could supply a special faster inline version for time critical code.
>
> Then it would be simple if (global_var != NULL) { ... } in the fast path.
> In addition the call chain could be declared __read_mostly.
>
> I suspect with these changes Christoph's concerns would go away, right?
>
> -Andi

So, assuming inlining the notifier_call_chain would address Christoph's
conserns, is the following patch something like what you are sugesting?
This would make all the kdebug.h::notify_die() calls use the inline version.

(WARNING: The following has only been tested on ia64.)

include/asm-i386/kdebug.h | 2 +-
include/asm-ia64/kdebug.h | 2 +-
include/asm-ppc64/kdebug.h | 2 +-
include/asm-sparc64/kdebug.h | 2 +-
include/asm-x86_64/kdebug.h | 2 +-
include/linux/notifier.h | 18 ++++++++++++++++++
kernel/sys.c | 14 +-------------
7 files changed, 24 insertions(+), 18 deletions(-)

Index: linux-2.6.13/include/linux/notifier.h
===================================================================
--- linux-2.6.13.orig/include/linux/notifier.h
+++ linux-2.6.13/include/linux/notifier.h
@@ -72,5 +72,23 @@ extern int notifier_call_chain(struct no
#define CPU_DOWN_FAILED 0x0006 /* CPU (unsigned)v NOT going down */
#define CPU_DEAD 0x0007 /* CPU (unsigned)v dead */

+static inline int fast_notifier_call_chain(struct notifier_block **n,
+ unsigned long val, void *v)
+{
+ int ret=NOTIFY_DONE;
+ struct notifier_block *nb = *n;
+
+ while(nb)
+ {
+ ret=nb->notifier_call(nb,val,v);
+ if(ret&NOTIFY_STOP_MASK)
+ {
+ return ret;
+ }
+ nb=nb->next;
+ }
+ return ret;
+}
+
#endif /* __KERNEL__ */
#endif /* _LINUX_NOTIFIER_H */
Index: linux-2.6.13/kernel/sys.c
===================================================================
--- linux-2.6.13.orig/kernel/sys.c
+++ linux-2.6.13/kernel/sys.c
@@ -169,19 +169,7 @@ EXPORT_SYMBOL(notifier_chain_unregister)

int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v)
{
- int ret=NOTIFY_DONE;
- struct notifier_block *nb = *n;
-
- while(nb)
- {
- ret=nb->notifier_call(nb,val,v);
- if(ret&NOTIFY_STOP_MASK)
- {
- return ret;
- }
- nb=nb->next;
- }
- return ret;
+ return fast_notifier_call_chain(n, val, v);
}

EXPORT_SYMBOL(notifier_call_chain);
Index: linux-2.6.13/include/asm-ia64/kdebug.h
===================================================================
--- linux-2.6.13.orig/include/asm-ia64/kdebug.h
+++ linux-2.6.13/include/asm-ia64/kdebug.h
@@ -55,7 +55,7 @@ static inline int notify_die(enum die_va
.signr = sig
};

- return notifier_call_chain(&ia64die_chain, val, &args);
+ return fast_notifier_call_chain(&ia64die_chain, val, &args);
}

#endif
Index: linux-2.6.13/include/asm-i386/kdebug.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/kdebug.h
+++ linux-2.6.13/include/asm-i386/kdebug.h
@@ -44,7 +44,7 @@ enum die_val {
static inline int notify_die(enum die_val val,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 notifier_call_chain(&i386die_chain, val, &args);
+ return fast_notifier_call_chain(&i386die_chain, val, &args);
}

#endif
Index: linux-2.6.13/include/asm-ppc64/kdebug.h
===================================================================
--- linux-2.6.13.orig/include/asm-ppc64/kdebug.h
+++ linux-2.6.13/include/asm-ppc64/kdebug.h
@@ -37,7 +37,7 @@ enum die_val {
static inline int notify_die(enum die_val val,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 notifier_call_chain(&ppc64_die_chain, val, &args);
+ return fast_notifier_call_chain(&ppc64_die_chain, val, &args);
}

#endif
Index: linux-2.6.13/include/asm-sparc64/kdebug.h
===================================================================
--- linux-2.6.13.orig/include/asm-sparc64/kdebug.h
+++ linux-2.6.13/include/asm-sparc64/kdebug.h
@@ -46,7 +46,7 @@ static inline int notify_die(enum die_va
.trapnr = trap,
.signr = sig };

- return notifier_call_chain(&sparc64die_chain, val, &args);
+ return fast_notifier_call_chain(&sparc64die_chain, val, &args);
}

#endif
Index: linux-2.6.13/include/asm-x86_64/kdebug.h
===================================================================
--- linux-2.6.13.orig/include/asm-x86_64/kdebug.h
+++ linux-2.6.13/include/asm-x86_64/kdebug.h
@@ -38,7 +38,7 @@ enum die_val {
static inline int notify_die(enum die_val val,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 notifier_call_chain(&die_chain, val, &args);
+ return fast_notifier_call_chain(&die_chain, val, &args);
}

extern int printk_address(unsigned long address);