linux-kernel - Re: [PATCH] mm/page_alloc: make calling prep_compound

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Message-ID: <YqnTtGDt+NdQ3Jxf@casper.infradead.org>
Date:   Wed, 15 Jun 2022 13:42:28 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     Joao Martins <joao.m.martins@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH] mm/page_alloc: make calling prep_compound_head more
 reliable

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.