Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Juergen Gross <jgross@suse.com>, Yi Sun <yi.y.sun@linux.intel.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
        chao.p.peng@intel.com, chao.gao@intel.com,
        isaku.yamahata@intel.com, michael.h.kelley@microsoft.com,
        tianyu.lan@microsoft.com, "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>
References: <1539954835-34035-1-git-send-email-yi.y.sun@linux.intel.com>
 <1539954835-34035-3-git-send-email-yi.y.sun@linux.intel.com>
 <bab010e9-a122-95f1-d8cc-790e56c72716@suse.com>
 <20181022015342.GK11769@yi.y.sun>
 <2e0d62cb-b706-a5b4-65f7-982a913fb32b@suse.com>
 <ba75b994-c1a6-6e91-6499-b9af90636fcf@redhat.com>
 <20181022171516.GH3117@worktop.programming.kicks-ass.net>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <6d8095c0-af95-5967-3ca5-2ceeb74233ea@redhat.com>
Date:   Mon, 22 Oct 2018 13:27:27 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.2.0
MIME-Version: 1.0
In-Reply-To: <20181022171516.GH3117@worktop.programming.kicks-ass.net>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8BIT
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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