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

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Date:   Tue, 5 Sep 2017 08:28:10 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Oscar Salvador <osalvador@...e.de>, 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 05/09/17 00:21, Davidlohr Bueso wrote:
> On Mon, 04 Sep 2017, Peter Zijlstra wrote:
> 
>> For testing its trivial to hack your kernel and I don't feel this is
>> something an Admin can make reasonable decisions about.
>>
>> So why? In general less knobs is better.
> 
> +1.
> 
> Also, note how b8fa70b51aa (xen, pvticketlocks: Add xen_nopvspin parameter
> to disable xen pv ticketlocks) has no justification as to why its wanted
> in the first place. The only thing I could find was from 15a3eac0784
> (xen/spinlock: Document the xen_nopvspin parameter):
> 
> "Useful for diagnosing issues and comparing benchmarks in over-commit
> CPU scenarios."

Hmm, I think I should clarify the Xen knob, as I was the one requesting
it:

In my previous employment we had a configuration where dom0 ran
exclusively on a dedicated set of physical cpus. We experienced
scalability problems when doing I/O performance tests: with a decent
number of dom0 cpus we achieved throughput of 700 MB/s with only 20%
cpu load in dom0. A higher dom0 cpu count let the throughput drop to
about 150 MB/s and cpu load was up to 100%. Reason was the additional
load due to hypervisor interactions on a high frequency lock.

So in special configurations at least for Xen the knob is useful for
production environment.

Juergen