On Fri, Jun 22, 2018 at 3:47 PM Thomas Gleixner <[email protected]> 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
