From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rafael Aquini Subject: Re: [PATCH] mm: expland documentation over __read_mostly Date: Wed, 6 May 2020 19:36:11 -0400 Message-ID: <20200506233611.GC205881@optiplex-lnx> References: <20200506231353.32451-1-mcgrof@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from us-smtp-2.mimecast.com ([207.211.31.81]:40929 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727121AbgEFXgW (ORCPT ); Wed, 6 May 2020 19:36:22 -0400 Content-Disposition: inline In-Reply-To: <20200506231353.32451-1-mcgrof@kernel.org> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Luis Chamberlain Cc: cl@linux.com, akpm@linux-foundation.org, arnd@arndb.de, willy@infradead.org, keescook@chromium.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org On Wed, May 06, 2020 at 11:13:53PM +0000, Luis Chamberlain 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. s/it/its same goes for the subject, as I think there is a minor typo: s/expland/expand > > Acked-by: Christoph Lameter > Signed-off-by: Luis Chamberlain > --- > > I sent this 2 years ago, but it fell through the cracks. This time > I'm adding Andrew Morton now, the fix0r-of-falling-through-the-cracks. > > Resending as I just saw a patch which doesn't clearly justifiy the > merits of the use of __read_mostly on it. > That would be my fault! (sorry) given the rationale below, the patch I sent really doesn't need the hint. Thanks for the extra bit of education here. (not an excuse) In a glance over the source tree, though, it seems most of the hinting cases are doing it in the misguided way. > include/linux/cache.h | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/include/linux/cache.h b/include/linux/cache.h > index 750621e41d1c..8106fb304fa7 100644 > --- a/include/linux/cache.h > +++ b/include/linux/cache.h > @@ -15,8 +15,14 @@ > > /* > * __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 > */ > #ifndef __read_mostly > #define __read_mostly > -- > 2.25.1 > Acked-by: Rafael Aquini