Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 9 Jun 2022 14:23:36 +0200 | Subject | Re: [PATCH 4/8] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable |
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On Thu, 9 Jun 2022 at 14:08, 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? > > Then there's little reason for the function either and we can just > directly increment/decrement 1 everywhere. If we drop the ability for > an arch to override, I feel that'd be cleaner. > > Either way, codegen won't change though. > > Preferences?
I don't have strong preferences either way. Can also be: #define HW_BREAKPOINT_WEIGHT 1
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