From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:38212)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <serge.fdrv@gmail.com>) id 1b37Uz-00075S-Jy
	for qemu-devel@nongnu.org; Wed, 18 May 2016 15:51:18 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <serge.fdrv@gmail.com>) id 1b37Uu-00083J-Jy
	for qemu-devel@nongnu.org; Wed, 18 May 2016 15:51:16 -0400
Received: from mail-lf0-x243.google.com ([2a00:1450:4010:c07::243]:34802)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <serge.fdrv@gmail.com>) id 1b37Uu-00083F-Bc
	for qemu-devel@nongnu.org; Wed, 18 May 2016 15:51:12 -0400
Received: by mail-lf0-x243.google.com with SMTP id m101so3881546lfi.1
	for <qemu-devel@nongnu.org>; Wed, 18 May 2016 12:51:12 -0700 (PDT)
References: <1463196873-17737-1-git-send-email-cota@braap.org>
	<1463196873-17737-8-git-send-email-cota@braap.org>
From: Sergey Fedorov <serge.fdrv@gmail.com>
Message-ID: <573CC7AD.2040108@gmail.com>
Date: Wed, 18 May 2016 22:51:09 +0300
MIME-Version: 1.0
In-Reply-To: <1463196873-17737-8-git-send-email-cota@braap.org>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH v5 07/18] qemu-thread: add simple
 test-and-set spinlock
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel/>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@listserver.greensocs.com>
Cc: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>

On 14/05/16 06:34, Emilio G. Cota wrote:
> +static inline void qemu_spin_lock(QemuSpin *spin)
> +{
> +    while (atomic_test_and_set_acquire(&spin->value)) {

A possible optimization might be using unlikely() here, copmare:

spin.o:     file format elf64-littleaarch64


Disassembly of section .text:

0000000000000000 <spin_lock__no_hint>:
   0:    52800022     mov    w2, #0x1                       // #1
   4:    885ffc01     ldaxr    w1, [x0]
   8:    88037c02     stxr    w3, w2, [x0]
   c:    35ffffc3     cbnz    w3, 4 <spin_lock__no_hint+0x4>
  10:    340000a1     cbz    w1, 24 <spin_lock__no_hint+0x24>
  14:    b9400001     ldr    w1, [x0]
  18:    34ffff61     cbz    w1, 4 <spin_lock__no_hint+0x4>
  1c:    d503203f     yield
  20:    17fffffd     b    14 <spin_lock__no_hint+0x14>
  24:    d65f03c0     ret

0000000000000028 <spin_lock__hint>:
  28:    52800022     mov    w2, #0x1                       // #1
  2c:    885ffc01     ldaxr    w1, [x0]
  30:    88037c02     stxr    w3, w2, [x0]
  34:    35ffffc3     cbnz    w3, 2c <spin_lock__hint+0x4>
  38:    35000061     cbnz    w1, 44 <spin_lock__hint+0x1c>
  3c:    d65f03c0     ret
  40:    d503203f     yield
  44:    b9400001     ldr    w1, [x0]
  48:    35ffffc1     cbnz    w1, 40 <spin_lock__hint+0x18>
  4c:    17fffff8     b    2c <spin_lock__hint+0x4>

spin_lock__hint(), the one where unlikely() used, gives a bit more
CPU-pipeline-friendly fast-path.

> +        while (atomic_read(&spin->value)) {
> +            cpu_relax();
> +        }
> +    }
> +}
> +
> +static inline int qemu_spin_trylock(QemuSpin *spin)
> +{
> +    if (atomic_test_and_set_acquire(&spin->value)) {
> +        return -EBUSY;
> +    }
> +    return 0;
> +}

Here we could also benefit from unlikely(), I think.

Kind regards,
Sergey