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

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Date:   Fri, 23 Dec 2016 16:41:53 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     kvm@...r.kernel.org
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, Chen Yu <yu.c.chen@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Radim Krcmar" <rkrcmar@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: [PATCH][RFC] VM: x86: Return ealier if clocksource has not changed

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@...hat.com>
Cc: "Radim Krcmar" <rkrcmar@...hat.com>
Cc: Marcelo Tosatti <mtosatti@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Wanpeng Li <wanpeng.li@...mail.com>
Signed-off-by: Chen Yu <yu.c.chen@...el.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