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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 9 Jun 2022 14:23:36 +0200
Message-ID: <CACT4Y+ayhGH253WA_zXYJjfOK=YxGLDZLzkyyqLHW+EZzJYpEg@mail.gmail.com>
Subject: Re: [PATCH 4/8] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable
To:     Marco Elver <elver@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-perf-users@vger.kernel.org, x86@kernel.org,
        linux-sh@vger.kernel.org, kasan-dev@googlegroups.com,
        linux-kernel@vger.kernel.org
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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