LKML Archive on
help / color / mirror / Atom feed
From: Andy Lutomirski <>
To: Thomas Gleixner <>,
	Stephen Boyd <>,
	John Stultz <>
Cc:, X86 ML <>,
	"H. Peter Anvin" <>, Ingo Molnar <>,
	LKML <>,
	Peter Zijlstra <>,
	Andrew Lutomirski <>
Subject: Re: [PATCH] arch/x86/entry/vsyscall/vsyscall_gtod.c: remove __read_mostly from vclocks_used
Date: Fri, 22 Jun 2018 16:25:34 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Fri, Jun 22, 2018 at 3:47 PM Thomas Gleixner <> wrote:
> Michael,
> On Mon, 4 Jun 2018, Michael Rodin wrote:
> > The variable "vclocks_used" doesn't appear to be "read mostly".
> > Measurements of the access frequency with perf stat [1] and
> > perf report show, that approximately half of the accesses to
> > this variable are write accesses and happen in update_vsyscall()
> > in the file arch/x86/entry/vsyscall/vsyscall_gtod.c.
> > The measurements were done with the kernel 4.13.0-43-generic used by
> > ubuntu as well as with the stable kernel 4.16.7 with a custom config.
> > I've used "perf bench sched" and iperf3 as workloads.
> >
> > This was discovered during my master thesis in the CADOS project [2].
> Nice find, but ...
> The point is that the content of that variable changes once in a blue moon,
> so the intent of marking it read_mostly is almost correct.

I would propose a rather different fix.  Add a an
arch_change_clocksource() function.  Do:

static inline void arch_change_clocksource(struct clocksource
*new_clocksource) { ... }
#define arch_change_clocksource arch_change_clocksource


#ifndef arch_change_clocksource
static inline void arch_change_clocksource(struct clocksource
*new_clocksource) {}

in the generic header.  In change_clocksource(), add a call to
arch_change_clocksource() right after tk_setup_internals().  In x86's
arch_change_clocksource, update vclocks_used.

Now it's genuinely read_mostly, and we don't need to touch that
cacheline at all in the normal clock tick code.  Everyone wins.
(vclocks_used is actually rather rarely read.  It's only used to
prevent user code from accessing a never-used clocksource through the
vvar area, which is a hardening measure.  It's only referenced from
the vvar fault handler code.)


  reply	other threads:[~2018-06-22 23:25 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-04 17:27 Michael Rodin
2018-06-22 22:47 ` Thomas Gleixner
2018-06-22 23:25   ` Andy Lutomirski [this message]
2018-06-23  9:04     ` Thomas Gleixner

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:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \ \
    --subject='Re: [PATCH] arch/x86/entry/vsyscall/vsyscall_gtod.c: remove __read_mostly from vclocks_used' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

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 NNTP newsgroup(s).