linux-kernel - Re: [PATCH v2] mm: expland documentation over __read

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Date:   Wed, 6 May 2020 17:03:15 -0600
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        Christopher Lamenter <cl@...ux.com>,
        Rafael Aquini <aquini@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Waiman Long <longman@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Randy Dunlap <rdunlap@...radead.org>, joel.opensrc@...il.com,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: expland documentation over __read_mostly

On Tue, May 8, 2018 at 12:19 PM Luis R. Rodriguez <mcgrof@...nel.org> wrote:
>
> __read_mostly can easily be misused by folks, its not meant for
> just read-only data. There are performance reasons for using it, but
> we also don't provide any guidance about its use. Provide a bit more
> guidance over it use.
>
> Acked-by: Christoph Lameter <cl@...ux.com>
> Signed-off-by: Luis R. Rodriguez <mcgrof@...nel.org>

After 2 years, this patch was never applied... and so people can
easily keep misusing this. I'll resend now.

  Luis
> ---
>  include/linux/cache.h | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/cache.h b/include/linux/cache.h
> index 750621e41d1c..4967566ed08c 100644
> --- a/include/linux/cache.h
> +++ b/include/linux/cache.h
> @@ -15,8 +15,16 @@
>
>  /*
>   * __read_mostly is used to keep rarely changing variables out of frequently
> - * updated cachelines. If an architecture doesn't support it, ignore the
> - * hint.
> + * updated cachelines. Its use should be reserved for data that is used
> + * frequently in hot paths. Performance traces can help decide when to use
> + * this. You want __read_mostly data to be tightly packed, so that in the
> + * best case multiple frequently read variables for a hot path will be next
> + * to each other in order to reduce the number of cachelines needed to
> + * execute a critial path. We should be mindful and selective of its use.
> + * ie: if you're going to use it please supply a *good* justification in your
> + * commit log.
> + *
> + * If an architecture doesn't support it, ignore the hint.
>   */
>  #ifndef __read_mostly
>  #define __read_mostly
> --
> 2.17.0
>