Re: [PATCH] arch/x86/entry/vsyscall/vsyscall_gtod.c: remove __read_mostly from vclocks_used

[Thomas Gleixner]

On Fri, 22 Jun 2018, Andy Lutomirski wrote:
> On Fri, Jun 22, 2018 at 3:47 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > 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.)

Agreed.

Thanks,

	tglx