linux-kernel - Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV spinlock

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Message-ID: <20170905080726.7qp752syjo7u5nmz@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Sep 2017 10:07:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Juergen Gross <jgross@...e.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Ingo Molnar <mingo@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV
 spinlock

On Tue, Sep 05, 2017 at 08:57:16AM +0200, Oscar Salvador wrote:
> It may be that the original patch was just to keep consistency between Xen
> and KVM, and also only for testing purposes.
> But we find a case when a customer of ours is running some workloads with
> 1<->1 mapping between physical cores and virtual cores, and we realized that
> with the pv spinlocks disabled there is a 4-5% of performance gain.

There are very definite downsides to using a test-and-set spinlock.

A much better option would be one that forces the use of native
qspinlock in the 1:1 case. That means you have to fail both pv_enabled()
and virt_spin_lock().