lkml.org 
[lkml]   [2018]   [Jun]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [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@linutronix.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

\
 
 \ /
  Last update: 2018-06-23 01:26    [W:0.057 / U:0.100 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site