From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp1.osuosl.org (smtp1.osuosl.org [140.211.166.138]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5CDC96AB3 for ; Sat, 30 Dec 2023 11:09:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="AIl9Y7yz" Received: from localhost (localhost [127.0.0.1]) by smtp1.osuosl.org (Postfix) with ESMTP id C40F481AC1 for ; Sat, 30 Dec 2023 11:09:26 +0000 (UTC) DKIM-Filter: OpenDKIM Filter v2.11.0 smtp1.osuosl.org C40F481AC1 Authentication-Results: smtp1.osuosl.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256 header.s=20230601 header.b=AIl9Y7yz X-Virus-Scanned: amavisd-new at osuosl.org X-Spam-Flag: NO X-Spam-Score: -1.5 X-Spam-Level: Received: from smtp1.osuosl.org ([127.0.0.1]) by localhost (smtp1.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id C1cWoEQdGbxD for ; Sat, 30 Dec 2023 11:09:26 +0000 (UTC) Received: from mail-wm1-x331.google.com (mail-wm1-x331.google.com [IPv6:2a00:1450:4864:20::331]) by smtp1.osuosl.org (Postfix) with ESMTPS id EFC4E81A24 for ; Sat, 30 Dec 2023 11:09:25 +0000 (UTC) DKIM-Filter: OpenDKIM Filter v2.11.0 smtp1.osuosl.org EFC4E81A24 Received: by mail-wm1-x331.google.com with SMTP id 5b1f17b1804b1-40d604b4b30so14702135e9.1 for ; Sat, 30 Dec 2023 03:09:25 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1703934564; x=1704539364; darn=lists.linux-foundation.org; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date:sender :from:to:cc:subject:date:message-id:reply-to; bh=IV2BKMSLhIOyXxbde9JcIy67hYaYu6fjXIgvjqlaklk=; b=AIl9Y7yz9fm6g9izCsegfrWgihJWUYcWaJNjofyzo7+2ZP3TJSlP/gUHO6AlIk0VF5 gUBch8y7472sVO4lh/iY2uAd6IffQB+96vvZgI0AmzNSkzQtRqscIbEuEN6EyDEy+qgj eavG0Y/A8cofvsthuUwiD//bCU/lD1IN6oJoxaaRUSfv7zQGIPI73JFUMbQUfEI7Qkq4 YvWsSUnstRnAka94OxxxM1iBNmhM9X5oc3ID+UumY6EEvOotikrBWfpj/mR1n3gEn1CS 5m/SGdnI+b4cgM6wTDfMUuZNXuTRbfQamZI0H1y2mQuMPRAVoKEVwX79m2bp57Wusmh9 kaag== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1703934564; x=1704539364; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date:sender :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=IV2BKMSLhIOyXxbde9JcIy67hYaYu6fjXIgvjqlaklk=; b=dpxiYJYusX1rtEPupqBXwqNkiHTntFTpf7FIpYHy3hUBWpW3la01fxmD658OHrWb2u 7zNBbooX7O9jg4Y/ZcYG/Z8cpS2+8/T2Wg2R8VjbwsmDiHFGxSoKYg7dxkla/3KvsIHN YeCBKZV2WlKSpqQva8FEYp1Tp8E4HxclLFZw+jAbjIM2Qv065PEGMe5NB8RjzdDNWX8H Nlc3v2K8Dh1msQXZK88HoknNjFKXLeAUtoavoeNrpJtkV/A2lho9WNySBpLZAErjwh3Z o0oEjbH8vBqSJKwKKlJxuVsdvktJ3wpzRR0+y1FhGx2nByk3jQA7hMcU7AENApE7X44O qbbg== X-Gm-Message-State: AOJu0YzamVPgg7yjsE7sg5vzIJpAv8pMUBc4i8gCx9qxgHj9Hpy8I7ZD Fzg6TO9QUnNGTQVsVyYAnwY= X-Google-Smtp-Source: AGHT+IHuqKd2XEDM9fpOy8gKESWCWJV0+bgGNvMKM9jUh8XBjrtBgKsh/lXXCzQqVstiQ9x3o0vSyQ== X-Received: by 2002:a05:600c:5398:b0:40d:418f:a66 with SMTP id hg24-20020a05600c539800b0040d418f0a66mr6147250wmb.7.1703934563783; Sat, 30 Dec 2023 03:09:23 -0800 (PST) Received: from gmail.com (195-38-113-95.pool.digikabel.hu. [195.38.113.95]) by smtp.gmail.com with ESMTPSA id p7-20020adfe607000000b00336843ae919sm21033294wrm.49.2023.12.30.03.09.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 30 Dec 2023 03:09:22 -0800 (PST) Sender: Ingo Molnar Date: Sat, 30 Dec 2023 12:09:20 +0100 From: Ingo Molnar To: Waiman Long Cc: David Laight , "'linux-kernel@vger.kernel.org'" , "'peterz@infradead.org'" , "'mingo@redhat.com'" , "'will@kernel.org'" , "'boqun.feng@gmail.com'" , 'Linus Torvalds' , "'xinhui.pan@linux.vnet.ibm.com'" , "'virtualization@lists.linux-foundation.org'" , 'Zeng Heng' Subject: Re: [PATCH next 4/5] locking/osq_lock: Optimise per-cpu data accesses. Message-ID: References: <73a4b31c9c874081baabad9e5f2e5204@AcuMS.aculab.com> <346c40b5-686f-461e-a1e3-9f255418efb2@redhat.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <346c40b5-686f-461e-a1e3-9f255418efb2@redhat.com> * Waiman Long wrote: > On 12/29/23 15:57, David Laight wrote: > > this_cpu_ptr() is rather more expensive than raw_cpu_read() since > > the latter can use an 'offset from register' (%gs for x86-84). > > > > Add a 'self' field to 'struct optimistic_spin_node' that can be > > read with raw_cpu_read(), initialise on first call. > > > > Signed-off-by: David Laight > > --- > > kernel/locking/osq_lock.c | 14 +++++++++----- > > 1 file changed, 9 insertions(+), 5 deletions(-) > > > > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c > > index 9bb3a077ba92..b60b0add0161 100644 > > --- a/kernel/locking/osq_lock.c > > +++ b/kernel/locking/osq_lock.c > > @@ -13,7 +13,7 @@ > > */ > > struct optimistic_spin_node { > > - struct optimistic_spin_node *next, *prev; > > + struct optimistic_spin_node *self, *next, *prev; > > int locked; /* 1 if lock acquired */ > > int cpu; /* encoded CPU # + 1 value */ > > }; > > @@ -93,12 +93,16 @@ osq_wait_next(struct optimistic_spin_queue *lock, > > bool osq_lock(struct optimistic_spin_queue *lock) > > { > > - struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); > > + struct optimistic_spin_node *node = raw_cpu_read(osq_node.self); > > My gcc 11 compiler produces the following x86-64 code: > > 92        struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); >    0x0000000000000029 <+25>:    mov    %rcx,%rdx >    0x000000000000002c <+28>:    add %gs:0x0(%rip),%rdx        # 0x34 > > > Which looks pretty optimized for me. Maybe older compiler may generate more > complex code. However, I do have some doubt as to the benefit of this patch > at the expense of making the code a bit more complex. GCC-11 is plenty of a look-back window in terms of compiler efficiency: latest enterprise distros use GCC-11 or newer, while recent desktop distros use GCC-13. Anything older won't matter, because no major distribution is going to use new kernels with old compilers. Thanks, Ingo