From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
To: perfbook@vger.kernel.org
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>, paulmck@kernel.org
Subject: [PATCH] memorder: Add info on recent x86 implemenation of smp_mb()
Date: Fri, 13 Oct 2023 01:22:38 +0000 [thread overview]
Message-ID: <20231013012238.1455140-1-joel@joelfernandes.org> (raw)
smp_mb() uses lock;add for x86 in the linux kernel. Add information
about the same.
Cc: paulmck@kernel.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
Not even build tested, just focused on the content and to keep my promise I'd
send this out (better than never sending it) ;-). I appreciate maintainers of
perfbook taking this forward ;-). Thanks!
bib/hw.bib | 8 ++++++++
memorder/memorder.tex | 8 ++++++++
2 files changed, 16 insertions(+)
diff --git a/bib/hw.bib b/bib/hw.bib
index b0885e74..b1dfd119 100644
--- a/bib/hw.bib
+++ b/bib/hw.bib
@@ -1159,3 +1159,11 @@ Luis Stevens and Anoop Gupta and John Hennessy",
note="\url{https://github.com/google/fuzzing/blob/master/docs/silifuzz.pdf}",
}
+@unpublished{Tsirkin2017,
+ Author="Michael S. Tsirkin",
+ Title="locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE",
+ month="November",
+ day="10",
+ year="2017",
+ note="\url{https://lore.kernel.org/all/tip-450cbdd0125cfa5d7bbf9e2a6b6961cc48d29730@git.kernel.org/}",
+}
diff --git a/memorder/memorder.tex b/memorder/memorder.tex
index 5c978fbe..b28ac4f0 100644
--- a/memorder/memorder.tex
+++ b/memorder/memorder.tex
@@ -6081,6 +6081,14 @@ A few older variants of the x86 CPU have a mode bit that enables out-of-order
stores, and for these CPUs, \co{smp_wmb()} must also be defined to
be \co{lock;addl}.
+A 2017 kernel commit by Michael S. Tsirkin replaced \co{mfence} with
+\co{lock add} in \co{smp_mb()}, achieving a 60 percent performance
+boost~\cite{Tsirkin2017}. The change used a 4-byte negative offset from
+the \co{SP} to avoid slowness due to false data-dependencies,
+instead of directly modifying the \co{SP}. \co{clflush} users still
+need to use \co{mfence} for ordering, so they have been converted to use
+\co{mb} instead of \co{smp_mb}, which uses an \co{mfence} as before.
+
Although newer x86 implementations accommodate self-modifying code
without any special instructions, to be fully compatible with
past and potential future x86 implementations, a given CPU must
--
2.42.0.655.g421f12c284-goog
next reply other threads:[~2023-10-13 1:22 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-13 1:22 Joel Fernandes (Google) [this message]
2023-10-14 3:07 ` [PATCH] memorder: Add info on recent x86 implemenation of smp_mb() Akira Yokosawa
2023-10-14 22:26 ` Joel Fernandes
2023-10-14 8:37 ` [PATCH -perfbook v2 0/3] memorder: Add info on recent x86 implementation " Akira Yokosawa
2023-10-14 8:39 ` [PATCH -perfbook v2 1/3] bib/memorymodel: Add Tsirkin2017 Akira Yokosawa
2023-10-14 8:42 ` [PATCH -perfbook v2 2/3] memorder: Add info on recent x86 implementation of smp_mb() Akira Yokosawa
2023-10-14 8:43 ` [PATCH -perfbook v2 3/3] memorder: Update of ordering SSE non-temporal memory move instructions Akira Yokosawa
2023-10-16 4:01 ` [PATCH -perfbook v2 0/3] memorder: Add info on recent x86 implementation of smp_mb() Paul E. McKenney
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20231013012238.1455140-1-joel@joelfernandes.org \
--to=joel@joelfernandes.org \
--cc=paulmck@kernel.org \
--cc=perfbook@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).