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

[Marco Elver <elver@google.com>]

On Thu, 9 Jun 2022 at 14:03, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> 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?

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.

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