[PATCH 4/8] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable

Marco Elver
2022-06-09T11:31:17Z

Due to being a __weak function, hw_breakpoint_weight() will cause the
compiler to always emit a call to it. This generates unnecessarily bad
code (register spills etc.) for no good reason; in fact it appears in
profiles of `perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512`:

...
0.70% [kernel] [k] hw_breakpoint_weight
...

While a small percentage, no architecture defines its own
hw_breakpoint_weight() nor are there users outside hw_breakpoint.c,
which makes the fact it is currently __weak a poor choice.

Change hw_breakpoint_weight()'s definition to follow a similar protocol
to hw_breakpoint_slots(), such that if <asm/hw_breakpoint.h> defines
hw_breakpoint_weight(), we'll use it instead.

The result is that it is inlined and no longer shows up in profiles.

Signed-off-by: Marco Elver <elver@google.com>
---
include/linux/hw_breakpoint.h | 1 -
kernel/events/hw_breakpoint.c | 4 +++-
2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
index 78dd7035d1e5..9fa3547acd87 100644
--- a/include/linux/hw_breakpoint.h
+++ b/include/linux/hw_breakpoint.h
@@ -79,7 +79,6 @@ extern int dbg_reserve_bp_slot(struct perf_event *bp);
extern int dbg_release_bp_slot(struct perf_event *bp);
extern int reserve_bp_slot(struct perf_event *bp);
extern void release_bp_slot(struct perf_event *bp);
-int hw_breakpoint_weight(struct perf_event *bp);
int arch_reserve_bp_slot(struct perf_event *bp);
void arch_release_bp_slot(struct perf_event *bp);
void arch_unregister_hw_breakpoint(struct perf_event *bp);
diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 8e939723f27d..5f40c8dfa042 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -125,10 +125,12 @@ static __init int init_breakpoint_slots(void)
}
#endif

-__weak int hw_breakpoint_weight(struct perf_event *bp)
+#ifndef hw_breakpoint_weight
+static inline int hw_breakpoint_weight(struct perf_event *bp)
{
return 1;
}
+#endif

static inline enum bp_type_idx find_slot_idx(u64 bp_type)
{
--
2.36.1.255.ge46751e96f-goog