linux-kernel - Re: [PATCH] select: do_pollfd: add unlikely branch hint return path

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Message-ID: <llt32u2qdjyu3giwhxesrahsh5a2ks6behzzkjky7fe7k6xync@pvixqbom73il>
Date: Wed, 9 Apr 2025 17:18:26 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Colin Ian King <colin.i.king@...il.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org, 
	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] select: do_pollfd: add unlikely branch hint return path

On Wed, Apr 09, 2025 at 03:31:38PM +0100, Colin Ian King wrote:
> Adding an unlikely() hint on the fd < 0 comparison return path improves
> run-time performance of the mincore system call. gcov based coverage
> analysis shows that this path return path is highly unlikely.
> 
> Benchmarking on an Debian based Intel(R) Core(TM) Ultra 9 285K with
> a 6.15-rc1 kernel and a poll of 1024 file descriptors with zero timeout
> shows an call reduction from 32818 ns down to 32635 ns, which is a ~0.5%
> performance improvement.
> 
> Results based on running 25 tests with turbo disabled (to reduce clock
> freq turbo changes), with 30 second run per test and comparing the number
> of poll() calls per second. The % standard deviation of the 25 tests
> was 0.08%, so results are reliable.
> 

I don't think adding a branch hint warrants benchmarking of the sort.

Instead the thing to do is to check if the prediction matches real world
uses.

While it is impossible to check this for all programs out there, it
should not be a significant time investment to look to check some of the
popular ones out there. Normally I would do it with bpftrace, but this
comes from a user-backed area instead of func args, so involved hackery
may be needed which is not warranted the change. Perhaps running strace
on a bunch of network progs would also do it (ssh, browser?).

I have to say I did not even know one can legally pass a fd < 0 to poll
and I never seen it in action, so I don't expect many users. ;)