Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.33 as permitted sender) client-ip=23.128.96.33;
MIME-Version: 1.0
References: <CAHk-=whM-cEwAsLtKsf5dYwV7nDTaRv1bUKLVBstMAQBug24uQ@mail.gmail.com>
 <CAHk-=wjCUckvZUQf7gqp2ziJUWxVpikM_6srFdbcNdBJTxExRg@mail.gmail.com>
 <CAHk-=wjhs6uuedgz-7HbcPtirEq+vvjJBY-M2zyteJwBhOMZhg@mail.gmail.com>
 <20231115190938.GGZVUXcuUjI3i1JRAB@fat_crate.local> <CAHk-=wh0TcXyGmKHfs+Xe=5Sd5bNn=NNV9CEtOy_tbyHAAmk9g@mail.gmail.com>
 <232440.1700153046@warthog.procyon.org.uk> <864270.1700230337@warthog.procyon.org.uk>
 <20231117160940.GGZVeQRLgLjJZXBLE1@fat_crate.local> <CAHk-=wj33FoGBQ7HkqjLbyOBQogWpYAG7WUTXatcfBF5duijjQ@mail.gmail.com>
 <CAHk-=whLbJ7vvB1ACVC6t44zjihX8w7GMY2y584+Fm83rsmaKg@mail.gmail.com>
 <20231117191243.GHZVe7K4vN9n5M92gb@fat_crate.local> <CAHk-=wiRQHD5xnB8H9Lwk9fJPDpfVNAwPS4KLnfrcrU3zbMAdQ@mail.gmail.com>
 <2284219.1700487177@warthog.procyon.org.uk>
In-Reply-To: <2284219.1700487177@warthog.procyon.org.uk>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 20 Nov 2023 08:06:30 -0800
Message-ID: <CAHk-=wid6dc9WpRdv5Gk1YL+bb9jkaMwc74b0UYA75tntq8xnw@mail.gmail.com>
Subject: Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput
 -16.9% regression
To:     David Howells <dhowells@redhat.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        kernel test robot <oliver.sang@intel.com>,
        oe-lkp@lists.linux.dev, lkp@intel.com,
        linux-kernel@vger.kernel.org,
        Christian Brauner <brauner@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Christian Brauner <christian@brauner.io>,
        Matthew Wilcox <willy@infradead.org>,
        David Laight <David.Laight@aculab.com>, ying.huang@intel.com,
        feng.tang@intel.com, fengwei.yin@intel.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Mon, 20 Nov 2023 at 05:33, David Howells <dhowells@redhat.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