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Date:   Wed, 15 Jun 2022 13:42:28 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Miaohe Lin <linmiaohe@huawei.com>
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Subject: Re: [PATCH] mm/page_alloc: make calling prep_compound_head more
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On Wed, Jun 15, 2022 at 03:44:06PM +0800, Miaohe Lin wrote:
> > We definitely don't need the unlikely here.
> 
> Could you please give me a more detailed explanation? IIUC, the above if condition
> will only meet at a probability of 1/512. So unlikely tells the compiler to do some
> optimization around it. Or am I miss something?

Only add unlikely() when the compiler can't figure out for itself that
it's unlikely.  You should also check the generated code and/or
benchmark the results to be sure that it's actually an improvement.
Using unlikely() needs to be backed up with more than just a feeling.