From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
To: Waiman Long <longman@redhat.com>, Juergen Gross <jgross@suse.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org, xen-devel@lists.xenproject.org,
linux-kernel@vger.kernel.org,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU
Date: Thu, 19 Jul 2018 18:35:46 -0400 [thread overview]
Message-ID: <2814c5d2-3016-8e9b-a644-0711fd92b9c7@oracle.com> (raw)
In-Reply-To: <1532036397-19449-1-git-send-email-longman@redhat.com>
On 07/19/2018 05:39 PM, Waiman Long wrote:
> 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@redhat.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.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;
next prev parent reply other threads:[~2018-07-19 22:35 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-19 21:39 [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU Waiman Long
2018-07-19 21:54 ` Davidlohr Bueso
2018-07-19 22:02 ` Waiman Long
2018-07-23 3:31 ` Wanpeng Li
2018-07-23 4:42 ` Davidlohr Bueso
2018-07-23 4:50 ` Davidlohr Bueso
2018-07-23 13:52 ` Waiman Long
2018-07-23 13:33 ` Waiman Long
2018-07-19 22:35 ` Boris Ostrovsky [this message]
2018-07-31 17:05 ` Boris Ostrovsky
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