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: Wed, 31 Oct 2018 09:54:17 +0800	[thread overview]
Message-ID: <20181031015417.GC15378@yi.y.sun> (raw)
In-Reply-To: <20181023093328.GA15378@yi.y.sun>

On 18-10-23 17:33:28, Yi Sun wrote:
> On 18-10-23 10:51:27, Peter Zijlstra wrote:
> > On Tue, Oct 23, 2018 at 10:57:40AM +0800, Yi Sun wrote:
> > > On 18-10-22 19:15:16, Peter Zijlstra wrote:
> > 
> > > > > >>>> +#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?
> > > > 
> > > Sorry for that. Will try another way.
> > 
> > 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.