linux-kernel - Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall

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Date:   Mon, 22 Oct 2018 14:01:47 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Juergen Gross <jgross@...e.com>, Yi Sun <yi.y.sun@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        chao.p.peng@...el.com, chao.gao@...el.com,
        isaku.yamahata@...el.com, michael.h.kelley@...rosoft.com,
        tianyu.lan@...rosoft.com, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall

On 10/22/2018 01:31 PM, Peter Zijlstra wrote:
> On Mon, Oct 22, 2018 at 01:27:27PM -0400, Waiman Long wrote:
>
>>>> I agree with Juergen on that. I would suggest rename the
>>>> vcpu_is_preempted hook into a more generic vcpu_stop_spinning, perhaps,
>>>> so different hypervisors can act on the information accordingly. Adding
>>>> an extra parameter is fine.
>>> No; no extra parameters. vcpu_is_preempted() is a simple and intuitive
>>> interface. Why would we want to make it complicated?
>> Hyperv seems to do it in a somewhat different way by looking at the spin
>> counter and decide if it should continue. I don't know why they do that
>> and what advantage it has.
>>
>> The current patch is definitely not OK. A revised patch that makes use
>> of an existing paravirt hook will be more acceptable. Again, I would
>> like to see some performance figure and why they do it this way to see
>> if it is worthwhile to change the existing interface.
> Note that there are vcpu_is_preempted() users that are not in a
> spin-loop.

You are right. I forgot about that. In that case, someone has to prove
that using an alternative way to stop spinning really has a performance
advantage compared to what we already have today.

Cheers,
Longman