linux-kernel - [PATCH] signal: optimise signal

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Message-Id: <a068c25c4c08aa8dbc1141a77614711f80f74b65.1621245687.git.asml.silence@gmail.com>
Date:   Mon, 17 May 2021 11:18:07 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH] signal: optimise signal_pending()

Optimise signal_pending() by checking both TIF_SIGPENDING and
TIF_NOTIFY_SIGNAL at once. Saves quite a bit of generated instructions,
e.g. sheds 240B from io_uring alone, some including ones in hot paths.

   text    data     bss     dec     hex filename
  84087   12414       8   96509   178fd ./fs/io_uring.o
  83847   12414       8   96269   1780d ./fs/io_uring.o

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---

Suggestions on how to make it less disruptive to abstractions are most
welcome, as even the one below fails to generated anything sane because
of test_bit()

return unlikely(test_ti_thread_flag(ti, TIF_SIGPENDING) |
		test_ti_thread_flag(ti, TIF_SIGPENDING));

 include/linux/sched/signal.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 3f6a0fcaa10c..97e1963a13fc 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -361,14 +361,14 @@ static inline int task_sigpending(struct task_struct *p)
 
 static inline int signal_pending(struct task_struct *p)
 {
+	struct thread_info *ti = task_thread_info(p);
+
 	/*
 	 * TIF_NOTIFY_SIGNAL isn't really a signal, but it requires the same
 	 * behavior in terms of ensuring that we break out of wait loops
 	 * so that notify signal callbacks can be processed.
 	 */
-	if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL)))
-		return 1;
-	return task_sigpending(p);
+	return unlikely(ti->flags & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL));
 }
 
 static inline int __fatal_signal_pending(struct task_struct *p)
-- 
2.31.1