Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] net: optimize cmpxchg in ip_idents_reserve
To:     David Miller <davem@davemloft.net>, zhangshaokun@hisilicon.com
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        jinyuqi@huawei.com, kuznet@ms2.inr.ac.ru, yoshfuji@linux-ipv6.org,
        edumazet@google.com, guoyang2@huawei.com
References: <1579058620-26684-1-git-send-email-zhangshaokun@hisilicon.com>
 <20200116.042722.153124126288244814.davem@davemloft.net>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <930faaff-4d18-452d-2e44-ef05b65dc858@gmail.com>
Date:   Thu, 16 Jan 2020 07:12:50 -0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.2.2
MIME-Version: 1.0
In-Reply-To: <20200116.042722.153124126288244814.davem@davemloft.net>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 1/16/20 4:27 AM, David Miller wrote:
> From: Shaokun Zhang <zhangshaokun@hisilicon.com>
> Date: Wed, 15 Jan 2020 11:23:40 +0800
> 
>> From: Yuqi Jin <jinyuqi@huawei.com>
>>
>> atomic_try_cmpxchg is called instead of atomic_cmpxchg that can reduce
>> the access number of the global variable @p_id in the loop. Let's
>> optimize it for performance.
>>
>> Cc: "David S. Miller" <davem@davemloft.net>
>> Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
>> Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
>> Cc: Eric Dumazet <edumazet@google.com>
>> Cc: Yang Guo <guoyang2@huawei.com>
>> Signed-off-by: Yuqi Jin <jinyuqi@huawei.com>
>> Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
> 
> I doubt this makes any measurable improvement in performance.
> 
> If you can document a specific measurable improvement under
> a useful set of circumstances for real usage, then put those
> details into the commit message and resubmit.
> 
> Otherwise, I'm not applying this, sorry.
> 


Real difference that could be made here is to 
only use this cmpxchg() dance for CONFIG_UBSAN

When CONFIG_UBSAN is not set, atomic_add_return() is just fine.

(Supposedly UBSAN should not warn about that either, but this depends on compiler version)