linux-kernel - Re: [PATCH] arch/x86/entry/vsyscall/vsyscall_gtod.c: remove __read_mostly from vclocks

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Date:   Fri, 22 Jun 2018 16:25:34 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        John Stultz <john.stultz@...aro.org>
Cc:     michael-git@...in.online, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] arch/x86/entry/vsyscall/vsyscall_gtod.c: remove
 __read_mostly from vclocks_used

On Fri, Jun 22, 2018 at 3:47 PM Thomas Gleixner <tglx@...utronix.de> 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

and

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

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.)

--Andy