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

From: Yi Sun <yi.y.sun@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Waiman Long <longman@redhat.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
Date: Thu, 1 Nov 2018 11:20:21 +0800	[thread overview]
Message-ID: <20181101032020.GE15378@yi.y.sun> (raw)
In-Reply-To: <20181031171538.GC13219@hirez.programming.kicks-ass.net>

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.

BTW, which performance test do you suggest? I am trying to test it by
AIM7.