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    Subject[tip: perf/core] perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable
    The following commit has been merged into the perf/core branch of tip:

    Commit-ID: 9caf87be118f4639537404eeb67dd444a3716e9a
    Gitweb: https://git.kernel.org/tip/9caf87be118f4639537404eeb67dd444a3716e9a
    Author: Marco Elver <elver@google.com>
    AuthorDate: Mon, 29 Aug 2022 14:47:12 +02:00
    Committer: Peter Zijlstra <peterz@infradead.org>
    CommitterDate: Tue, 30 Aug 2022 10:56:22 +02:00

    perf/hw_breakpoint: Make hw_breakpoint_weight() inlinable

    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>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
    Acked-by: Ian Rogers <irogers@google.com>
    Link: https://lore.kernel.org/r/20220829124719.675715-8-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 a3fb846..f319bd2 100644
    --- a/include/linux/hw_breakpoint.h
    +++ b/include/linux/hw_breakpoint.h
    @@ -80,7 +80,6 @@ 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 9fb66d3..9c9bf17 100644
    --- a/kernel/events/hw_breakpoint.c
    +++ b/kernel/events/hw_breakpoint.c
    @@ -124,10 +124,12 @@ err:
    }
    #endif

    -__weak int hw_breakpoint_weight(struct perf_event *bp)
    +#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)
    {
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