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[73.241.150.58]) by smtp.gmail.com with ESMTPSA id p28sm25157336pgb.93.2020.01.16.07.12.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 16 Jan 2020 07:12:52 -0800 (PST) Subject: Re: [PATCH] net: optimize cmpxchg in ip_idents_reserve To: David Miller , 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 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: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 1/16/20 4:27 AM, David Miller wrote: > From: Shaokun Zhang > Date: Wed, 15 Jan 2020 11:23:40 +0800 > >> From: Yuqi Jin >> >> 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" >> Cc: Alexey Kuznetsov >> Cc: Hideaki YOSHIFUJI >> Cc: Eric Dumazet >> Cc: Yang Guo >> Signed-off-by: Yuqi Jin >> Signed-off-by: Shaokun Zhang > > 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)