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

[Marcelo Tosatti]

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@xxxxxxxxxx>
> Cc: "Radim Krcmar" <rkrcmar@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> ---
>  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.

>  	update_pvclock_gtod(tk);
>  
>  	/* disable master clock if host does not trust, or does not
>  	 * use, TSC clocksource
>  	 */
> -	if (gtod->clock.vclock_mode != VCLOCK_TSC &&
> -	    atomic_read(&kvm_guest_has_master_clock) != 0)
> +	if (atomic_read(&kvm_guest_has_master_clock) != 0)
>  		queue_work(system_long_wq, &pvclock_gtod_work);
>  
>  	return 0;
> -- 
> 2.7.4