Re: [PATCH v2 06/13] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable

Dmitry Vyukov
2022-06-28T13:17:44Z

On Tue, 28 Jun 2022 at 11:59, Marco Elver <elver@google.com> wrote:
>
> 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>

Reviewed-by: Dmitry Vyukov <dvyukov@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 a089302ddf59..a124786e3ade 100644
> --- 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)
> {
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>