From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Dmitry Safonov <dima@arista.com>
Cc: linux-kernel@vger.kernel.org,
Dmitry Safonov <0x7f454c46@gmail.com>,
Adrian Reber <adrian@lisas.de>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Andrei Vagin <avagin@openvz.org>,
Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
Oleg Nesterov <oleg@redhat.com>,
Pavel Emelyanov <xemul@virtuozzo.com>,
Thomas Gleixner <tglx@linutronix.de>,
containers@lists.linux-foundation.org,
linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 4/9] select: Micro-optimise __estimate_accuracy()
Date: Mon, 9 Sep 2019 14:18:12 +0300 [thread overview]
Message-ID: <20190909111812.GB1508@uranus> (raw)
In-Reply-To: <20190909102340.8592-5-dima@arista.com>
On Mon, Sep 09, 2019 at 11:23:35AM +0100, Dmitry Safonov wrote:
> Shift on s64 is faster than division, use it instead.
>
> As the result of the patch there is a hardly user-visible effect:
> poll(), select(), etc syscalls will be a bit more precise on ~2.3%
> than before because 1000 != 1024 :)
>
> Signed-off-by: Dmitry Safonov <dima@arista.com>
> ---
> fs/select.c | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/fs/select.c b/fs/select.c
> index 12cdefd3be2d..2477c202631e 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -51,15 +51,14 @@
>
> static long __estimate_accuracy(ktime_t slack)
> {
> - int divfactor = 1000;
> -
> if (slack < 0)
> return 0;
>
> - if (task_nice(current) > 0)
> - divfactor = divfactor / 5;
> + /* A bit more precise than 0.1% */
> + slack = slack >> 10;
>
> - slack = ktime_divns(slack, divfactor);
> + if (task_nice(current) > 0)
> + slack = slack * 5;
>
> if (slack > MAX_SLACK)
> return MAX_SLACK;
Compiler precompute constants so it doesn't do division here.
But I didn't read the series yet so I might be missing
something obvious.
next prev parent reply other threads:[~2019-09-09 11:18 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-09 10:23 [PATCH 0/9] restart_block: Prepare the ground for dumping timeout Dmitry Safonov
2019-09-09 10:23 ` [PATCH 1/9] futex: Remove unused uaddr2 in restart_block Dmitry Safonov
2019-09-09 10:23 ` [PATCH 2/9] restart_block: Prevent userspace set part of the block Dmitry Safonov
2019-09-09 10:23 ` [PATCH 3/9] select: Convert __esimate_accuracy() to ktime_t Dmitry Safonov
2019-09-09 10:23 ` [PATCH 4/9] select: Micro-optimise __estimate_accuracy() Dmitry Safonov
2019-09-09 11:18 ` Cyrill Gorcunov [this message]
2019-09-09 11:50 ` Dmitry Safonov
2019-09-09 12:14 ` Cyrill Gorcunov
2019-09-19 14:05 ` Cyrill Gorcunov
2019-09-19 14:25 ` Dmitry Safonov
2019-09-09 10:23 ` [PATCH 5/9] select: Convert select_estimate_accuracy() to take ktime_t Dmitry Safonov
2019-09-09 10:23 ` [PATCH 6/9] select: Extract common code into do_sys_ppoll() Dmitry Safonov
2019-09-09 11:15 ` kbuild test robot
2019-09-09 19:48 ` kbuild test robot
2019-09-09 10:23 ` [PATCH 7/9] select: Use ktime_t in do_sys_poll() and do_poll() Dmitry Safonov
2019-09-09 10:23 ` [PATCH 8/9] select/restart_block: Convert poll's timeout to u64 Dmitry Safonov
2019-09-09 13:07 ` David Laight
2019-09-16 15:19 ` Dmitry Safonov
2019-09-09 10:23 ` [PATCH 9/9] restart_block: Make common timeout Dmitry Safonov
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