linux-kernel - Re: [PATCH] block: Remove unnecessary unlikely()

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Date: Fri, 19 Jan 2024 13:20:05 -0700
From: Jens Axboe <axboe@...nel.dk>
To: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: stable@...r.kernel.org
Subject: Re: [PATCH] block: Remove unnecessary unlikely()

On 1/19/24 9:41 AM, Jens Axboe wrote:
> On 1/19/24 9:34 AM, Matthew Wilcox (Oracle) wrote:
>> Jens added unlikely() thinking that this was an error path.  It's
>> actually just the end of the iteration, so does not warrant an
>> unlikely().
> 
> This is because the previous fix (or my attempt at least) didn't do the
> i >= vcnt, it checked for an empty bio instead. Which then definitely
> did make it an error/unlikely path, but obviously this one is not.

Just out of curiosity, I did some branch profiling on just normal
operations of on my box. Of the ~900K times we hit this path,
10% of them ended up in that branch, and 90% of them did not.
While it's not an error path, that does seem rather unlikely. Sure, for
single entries, it'll be hit 50% of the time, but for most normal IO
it'd definitely be less than 50%, and as per above non-scientif
profiling, it's around 10%.

-- 
Jens Axboe