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Date:   Wed, 31 Oct 2018 18:15:39 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Yi Sun <yi.y.sun@linux.intel.com>, Juergen Gross <jgross@suse.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>
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall
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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?