linux-kernel - Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput -16.9% regression

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Message-ID: <CAHk-=wid6dc9WpRdv5Gk1YL+bb9jkaMwc74b0UYA75tntq8xnw@mail.gmail.com>
Date:   Mon, 20 Nov 2023 08:06:30 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        kernel test robot <oliver.sang@...el.com>,
        oe-lkp@...ts.linux.dev, lkp@...el.com,
        linux-kernel@...r.kernel.org,
        Christian Brauner <brauner@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        Christian Brauner <christian@...uner.io>,
        Matthew Wilcox <willy@...radead.org>,
        David Laight <David.Laight@...lab.com>, ying.huang@...el.com,
        feng.tang@...el.com, fengwei.yin@...el.com
Subject: Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput
 -16.9% regression

On Mon, 20 Nov 2023 at 05:33, David Howells <dhowells@...hat.com> wrote:
>
> So what, if anything, should I change?

I don't think you need to worry about this.

Not that I like ignoring kernel robot report performance regressions,
because they've often been useful to find unexpected issues. But this
one seems to clearly be just a random code choice issue by the
compiler, and be very CPU-specific anyway.

We'll figure out some good way to make memcpy() a bit more reliable
wrt the code it generates, but it's not the iov_iter code that should
worry about it.

               Linus