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

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

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;
 	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