linux-kernel - [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU

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Date:   Thu, 19 Jul 2018 17:39:57 -0400
From:   Waiman Long <longman@...hat.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU

On a VM with only 1 vCPU, the locking fast paths will always be
successful. In this case, there is no need to use the the PV qspinlock
code which has higher overhead on the unlock side than the native
qspinlock code.

The xen_pvspin veriable is also turned off in this 1 vCPU case to
eliminate unneeded pvqspinlock initialization in xen_init_lock_cpu()
which is run after xen_init_spinlocks().

Signed-off-by: Waiman Long <longman@...hat.com>
---
 arch/x86/xen/spinlock.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index cd97a62..973f10e 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -130,6 +130,10 @@ void xen_uninit_lock_cpu(int cpu)
 void __init xen_init_spinlocks(void)
 {
 
+	/*  Don't need to use pvqspinlock code if there is only 1 vCPU. */
+	if (num_possible_cpus() == 1)
+		xen_pvspin = false;
+
 	if (!xen_pvspin) {
 		printk(KERN_DEBUG "xen: PV spinlocks disabled\n");
 		return;
-- 
1.8.3.1