linux-kernel - [PATCH 4/8] perf/hw_breakpoint: Make hw_breakpoint

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Message-Id: <20220609113046.780504-5-elver@google.com>
Date:   Thu,  9 Jun 2022 13:30:42 +0200
From:   Marco Elver <elver@...gle.com>
To:     elver@...gle.com, Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-perf-users@...r.kernel.org, x86@...nel.org,
        linux-sh@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: [PATCH 4/8] 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@...gle.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)
+#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)
 {
-- 
2.36.1.255.ge46751e96f-goog