netdev - Re: [PATCH] net: optimize cmpxchg in ip_idents

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Message-ID: <930faaff-4d18-452d-2e44-ef05b65dc858@gmail.com>
Date:   Thu, 16 Jan 2020 07:12:50 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Miller <davem@...emloft.net>, zhangshaokun@...ilicon.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        jinyuqi@...wei.com, kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        edumazet@...gle.com, guoyang2@...wei.com
Subject: Re: [PATCH] net: optimize cmpxchg in ip_idents_reserve



On 1/16/20 4:27 AM, David Miller wrote:
> From: Shaokun Zhang <zhangshaokun@...ilicon.com>
> Date: Wed, 15 Jan 2020 11:23:40 +0800
> 
>> From: Yuqi Jin <jinyuqi@...wei.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@...emloft.net>
>> Cc: Alexey Kuznetsov <kuznet@....inr.ac.ru>
>> Cc: Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
>> Cc: Eric Dumazet <edumazet@...gle.com>
>> Cc: Yang Guo <guoyang2@...wei.com>
>> Signed-off-by: Yuqi Jin <jinyuqi@...wei.com>
>> Signed-off-by: Shaokun Zhang <zhangshaokun@...ilicon.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)