On 18-11-01 08:59:08, Waiman Long wrote: > On 10/31/2018 11:20 PM, Yi Sun wrote: > > 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. > > I am hesitant to add any additional check at the spinning loop in > pv_wait_head_or_lock() especially one that is a hypercall or a callback > that will take time to execute. The testing that I had done in the past > indicated that it would slow down locking performance especially if the > VM wasn't overcommitted at all. > > Any additional slack in pv_wait_node() can be mitigated by the lock > stealing that can happen. Slack in pv_wait_head_or_lock(), on the other > hand, will certainly increase the lock transfer latency and impact > performance. So you need performance data to show that it is worthwhile > to do so. > Ok, I will make performance test to show if it is worthwhile to call SpinWaitInfo in pv_wait_head_or_lock.
> As for performance test, the kernel has a builtin locktorture test if > you configured it in. So show us the performance data with and without > the patch. Thank you! I will make performance test for whole patch. > > Cheers, > Longman
