linux-kernel - Re: [PATCH] signal: optimise signal

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Date:   Mon, 17 May 2021 12:22:13 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] signal: optimise signal_pending()

Pavel Begunkov <asml.silence@...il.com> writes:

> 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

I believe the atomic test_bit is pretty fundamental, especially with
it's implied barriers.  I believe you are optimizing out the code
that will makes signal_pending work in a loop.

I have tried looking and I really don't understand why TIF_NOTIFY_SIGNAL
was added.  Perhaps instead of trying to optimize the test, you should
optimize by combining TIF_NOTIFY_SIGNAL with TIF_SIGPENDING.

Perhaps set_notify_signal could be optimized to set both.  I think I
only see 4 calls in the tree.

> 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)