linux-kernel - [tip: perf/core] perf/hw_breakpoint: Make hw_breakpoint

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Date:   Thu, 01 Sep 2022 08:12:46 -0000
From:   "tip-bot2 for Marco Elver" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Marco Elver <elver@...gle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ian Rogers <irogers@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
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@...gle.com>
AuthorDate:    Mon, 29 Aug 2022 14:47:12 +02:00
Committer:     Peter Zijlstra <peterz@...radead.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@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
Acked-by: Ian Rogers <irogers@...gle.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)
 {