From: Yi Sun <email@example.com> To: Peter Zijlstra <firstname.lastname@example.org> Cc: Waiman Long <email@example.com>, Juergen Gross <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, "K. Y. Srinivasan" <email@example.com>, Haiyang Zhang <firstname.lastname@example.org>, Stephen Hemminger <email@example.com>, "firstname.lastname@example.org" <email@example.com>, Will Deacon <firstname.lastname@example.org> 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.
next prev parent reply other threads:[~2018-11-01 3:23 UTC|newest] Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-10-19 13:13 [PATCH v1 0/2] Enable HvNotifyLongSpinWait for Hyper-V Yi Sun 2018-10-19 13:13 ` [PATCH v1 1/2] x86/hyperv: get spinlock retry number on Hyper-V Yi Sun 2018-10-19 13:13 ` [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall Yi Sun 2018-10-19 14:20 ` Juergen Gross 2018-10-22 1:53 ` Yi Sun 2018-10-22 7:32 ` Juergen Gross 2018-10-22 16:31 ` Waiman Long 2018-10-22 17:15 ` Peter Zijlstra 2018-10-22 17:27 ` Waiman Long 2018-10-22 17:31 ` Peter Zijlstra 2018-10-22 18:01 ` Waiman Long 2018-10-23 2:57 ` Yi Sun 2018-10-23 8:51 ` Peter Zijlstra 2018-10-23 9:33 ` Yi Sun 2018-10-31 1:54 ` Yi Sun 2018-10-31 14:10 ` Peter Zijlstra 2018-10-31 15:07 ` Waiman Long 2018-10-31 17:15 ` Peter Zijlstra 2018-11-01 3:20 ` Yi Sun [this message] 2018-11-01 8:59 ` Peter Zijlstra 2018-11-01 12:59 ` Waiman Long 2018-11-05 6:54 ` Yi Sun 2018-10-24 16:53 ` Michael Kelley 2018-10-25 2:23 ` Yi Sun 2018-10-31 2:06 ` Yi Sun
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