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Subject: Re: [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by removing
 preemption on/off
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> I get:
>
> 		mov    %gs:0x18(%rax),%rdx
>
> Looks to me that %gs is used.

%gs is used as a segment prefix. That does not add significant cycles.
Retrieving the content of %gs and loading it into another register would
be expensive in terms of cpu cycles.