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

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Date:   Mon, 22 Oct 2018 13:27:27 -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 01:15 PM, Peter Zijlstra wrote:
> Firstly, who come a patch that is grubbing around in kernel/locking/ has
> an x86/hyperv subject and isn't Cc'ed to the locking maintainers?
>
> On Mon, Oct 22, 2018 at 12:31:45PM -0400, Waiman Long wrote:
>> On 10/22/2018 03:32 AM, Juergen Gross wrote:
>>> On 22/10/2018 03:53, Yi Sun wrote:
>>>> On 18-10-19 16:20:52, Juergen Gross wrote:
>>>>> On 19/10/2018 15:13, Yi Sun wrote:
>>>> [...]
>>>>
>>>>>> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
>>>>>> index 0130e48..9e88c7e 100644
>>>>>> --- a/kernel/locking/qspinlock_paravirt.h
>>>>>> +++ b/kernel/locking/qspinlock_paravirt.h
>>>>>> @@ -7,6 +7,8 @@
>>>>>>  #include <linux/bootmem.h>
>>>>>>  #include <linux/debug_locks.h>
>>>>>>  
>>>>>> +#include <asm/mshyperv.h>
>>>>>> +
>>>>>>  /*
>>>>>>   * Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
>>>>>>   * of spinning them.
>>>>>> @@ -305,6 +307,10 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
>>>>>>  				wait_early = true;
>>>>>>  				break;
>>>>>>  			}
>>>>>> +#if defined(CONFIG_X86_64) && defined(CONFIG_PARAVIRT_SPINLOCKS) && IS_ENABLED(CONFIG_HYPERV)
>>>>>> +			if (!hv_notify_long_spin_wait(SPIN_THRESHOLD - loop))
>>>>>> +				break;
>>>>>> +#endif
> Secondly; how come you thought that was acceptable in any way shape or
> form?
>
>>> vcpu_is_preempted() is already part of this loop. And this is a paravirt
>>> hook. Can't you make use of that? This might require adding another
>>> parameter to this hook, but I'd prefer that over another pv-spinlock
>>> hook.
>> 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.

Cheers,
Longman