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

[Yi Sun]

On 18-11-01 08:59:08, Waiman Long wrote:
> On 10/31/2018 11:20 PM, Yi Sun wrote:
> > On 18-10-31 18:15:39, Peter Zijlstra wrote:
> >> On Wed, Oct 31, 2018 at 11:07:22AM -0400, Waiman Long wrote:
> >>> On 10/31/2018 10:10 AM, Peter Zijlstra wrote:
> >>>> On Wed, Oct 31, 2018 at 09:54:17AM +0800, Yi Sun wrote:
> >>>>> On 18-10-23 17:33:28, Yi Sun wrote:
> >>>>>> On 18-10-23 10:51:27, Peter Zijlstra wrote:
> >>>>>>> Can you try and explain why vcpu_is_preempted() doesn't work for you?
> >>>>>> I thought HvSpinWaitInfo is used to notify hypervisor the spin number
> >>>>>> which is different with vcpu_is_preempted. So I did not consider
> >>>>>> vcpu_is_preempted.
> >>>>>>
> >>>>>> But HvSpinWaitInfo is a quite simple function and could be combined
> >>>>>> with vcpu_is_preempted together. So I think it is OK to use
> >>>>>> vcpu_is_preempted to make codes clean. I will have a try.
> >>>>> After checking codes, there is one issue to call vcpu_is_preempted.
> >>>>> There are two spin loops in qspinlock_paravirt.h. One loop in
> >>>>> 'pv_wait_node' calls vcpu_is_preempted. But another loop in
> >>>>> 'pv_wait_head_or_lock' does not call vcpu_is_preempted. It also does
> >>>>> not call any other ops of 'pv_lock_ops' in the loop. So I am afraid
> >>>>> we have to add one more ops in 'pv_lock_ops' to do this.
> >>>> Why? Would not something like the below cure that? Waiman, can you have
> >>>> a look at this; I always forget how that paravirt crud works.
> >>> There are two major reasons why the vcpu_is_preempt() test isn't done at
> >>> pv_wait_head_or_lock(). First of all, we may not have a valid prev
> >>> pointer after all if it is the first one to enter the queue while the
> >>> lock is busy. Secondly, because of lock stealing, the cpu number pointed
> >>> by a valid prev pointer may not be the actual cpu that is currently
> >>> holding the lock. Another minor reason is that we want to minimize the
> >>> lock transfer latency and so don't want to sleep too early while waiting
> >>> at the queue head.
> >> So Yi, are you actually seeing a problem? If so, can you give details?
> > Where does the patch come from? I cannot find it through google.
> >
> > Per Waiman's comment, it seems not suitable to call vcpu_is_preempted()
> > in pv_wait_head_or_lock(). So, we cannot make HvSpinWaitInfo notification
> > through vcpu_is_preempted() for such case. Based on that, I suggest to
> > add one more callback function in pv_lock_ops.
> 
> I am hesitant to add any additional check at the spinning loop in
> pv_wait_head_or_lock() especially one that is a hypercall or a callback
> that will take time to execute. The testing that I had done in the past
> indicated that it would slow down locking performance especially if the
> VM wasn't overcommitted at all.
> 
> Any additional slack in pv_wait_node() can be mitigated by the lock
> stealing that can happen. Slack in pv_wait_head_or_lock(), on the other
> hand, will certainly increase the lock transfer latency and impact
> performance. So you need performance data to show that it is worthwhile
> to do so.
> 
Ok, I will make performance test to show if it is worthwhile to call
SpinWaitInfo in pv_wait_head_or_lock.

> As for performance test, the kernel has a builtin locktorture test if
> you configured it in. So show us the performance data with and without
> the patch.

Thank you! I will make performance test for whole patch.

> 
> Cheers,
> Longman