Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 22 Jun 2018 16:25:34 -0700 | 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@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
| |