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

Dmitry Vyukov
2022-06-09T12:23:57Z

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