perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable

melver · Peter Zijlstra · commit 9caf87be118f · 2022-08-30T10:56:22.000+02:00
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> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Dmitry Vyukov <dvyukov@google.com> Acked-by: Ian Rogers <irogers@google.com> Link: https://lore.kernel.org/r/20220829124719.675715-8-elver@google.com
diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
@@ -80,7 +80,6 @@ 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
@@ -124,10 +124,12 @@ 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)
 {

Original file line number	Diff line number	Diff line change
`@@ -124,10 +124,12 @@ static __init int init_breakpoint_slots(void)`
`124`	`124`	`}`
`125`	`125`	`#endif`
`126`	`126`
`127`		`-__weak int hw_breakpoint_weight(struct perf_event *bp)`
	`127`	`+#ifndef hw_breakpoint_weight`
	`128`	`+static inline int hw_breakpoint_weight(struct perf_event *bp)`
`128`	`129`	`{`
`129`	`130`	`return 1;`
`130`	`131`	`}`
	`132`	`+#endif`
`131`	`133`
`132`	`134`	`static inline enum bp_type_idx find_slot_idx(u64 bp_type)`
`133`	`135`	`{`