Re: [PATCH][RFC] VM: x86: Return ealier if clocksource has not changed

[Chen Yu <yu.c.chen@intel.com>]

On Tue, Dec 27, 2016 at 01:32:47PM -0200, Marcelo Tosatti wrote:
> On Tue, Dec 27, 2016 at 04:06:44PM +0800, Chen Yu wrote:
> > Hi Marcelo,
> > On Mon, Dec 26, 2016 at 05:44:25PM -0200, Marcelo Tosatti wrote:
> > > On Fri, Dec 23, 2016 at 04:41:53PM +0800, Chen Yu wrote:
> > > > Currently the notifier of pvclock_gtod_notify() get invoked
> > > > frequently due to the periodic update_wall_time(). This might
> > > > slow down the system a little bit as there might be redundant
> > > > execution code path and unnecessary lock contention
> > > > in update_pvclock_gtod(), which was found when I was doing
> > > > suspend/resume speed testings. As pvclock_gtod_notify()
> > > > should be invoked only when clocksource has changed, according to
> > > > Commit 16e8d74d2da9 ("KVM: x86: notifier for clocksource changes")
> > > > , either we can add a new notifier for clocksource switch,
> > > > or we can simply bypass the following code in pvclock_gtod_notify()
> > > > earlier if there is no clocksource switch.
> > > > 
> > > > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > > Cc: "Radim Krcmar" <rkrcmar@redhat.com>
> > > > Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > > Cc: Ingo Molnar <mingo@redhat.com>
> > > > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > > > Cc: Wanpeng Li <wanpeng.li@hotmail.com>
> > > > Signed-off-by: Chen Yu <yu.c.chen@intel.com>
> > > > ---
> > > >  arch/x86/kvm/x86.c | 5 +++--
> > > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > index 445c51b..54aa32d 100644
> > > > --- a/arch/x86/kvm/x86.c
> > > > +++ b/arch/x86/kvm/x86.c
> > > > @@ -5961,13 +5961,14 @@ static int pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
> > > >  	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
> > > >  	struct timekeeper *tk = priv;
> > > >  
> > > > +	if (likely(gtod->clock.vclock_mode == VCLOCK_TSC))
> > > > +		return 0;
> > > 
> > > I think this is only safe if any of the values in "struct
> > > pvclock_gtod_data" are unchanged. Otherwise the local (KVM) copy is 
> > > kept incorrect.
> > I missread the code previously and I thought only under the condition
> > the clocksource has been switched to another one will the KVM copy
> > be touched. Apparently it is not the case because the copy should
> > be updated on-time during normal tick, right?
> > thanks for your reply,
> 
> Yes, it is updated during the normal tick, and mult/freq values change.
> 
> However, if none of them change, its not necessary to call the callback.
> Perhaps you can check if any of the values changed and only 
> invoke the callback in that case?
>
Yes, this should be an optimization, but most of the callers(workload) come
from update_wall_time(), and in this code path the clock source's cycle
should already be updated in most cases, so this optimization should not take
much effect to reduce the burden I guess?

Thanks,
Yu