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Y. Srinivasan" , Haiyang Zhang , Stephen Hemminger , "mingo@redhat.com" , Will Deacon Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall Message-ID: <20181031171538.GC13219@hirez.programming.kicks-ass.net> References: <20181022015342.GK11769@yi.y.sun> <2e0d62cb-b706-a5b4-65f7-982a913fb32b@suse.com> <20181022171516.GH3117@worktop.programming.kicks-ass.net> <20181023025740.GL11769@yi.y.sun> <20181023085127.GG3109@worktop.c.hoisthospitality.com> <20181023093328.GA15378@yi.y.sun> <20181031015417.GC15378@yi.y.sun> <20181031141030.GB13219@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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?