Re: [PATCH] net: optimize cmpxchg in ip_idents_reserve

[Eric Dumazet]

On 1/16/20 4:27 AM, David Miller wrote:
> From: Shaokun Zhang <zhangshaokun@xxxxxxxxxxxxx>
> Date: Wed, 15 Jan 2020 11:23:40 +0800
> 
>> From: Yuqi Jin <jinyuqi@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxx>
>> Cc: Alexey Kuznetsov <kuznet@xxxxxxxxxxxxx>
>> Cc: Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>
>> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
>> Cc: Yang Guo <guoyang2@xxxxxxxxxx>
>> Signed-off-by: Yuqi Jin <jinyuqi@xxxxxxxxxx>
>> Signed-off-by: Shaokun Zhang <zhangshaokun@xxxxxxxxxxxxx>
> 
> 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)