Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 9 Jun 2022 14:03:12 +0200 | Subject | Re: [PATCH 4/8] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable |
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On Thu, 9 Jun 2022 at 13:31, 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> > --- > 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)
Humm... this was added in 2010 and never actually used to return anything other than 1 since then (?). Looks like over-design. Maybe we drop "#ifndef" and add a comment instead?
> +#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 >
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