On Thu, Mar 23, 2023 at 01:17:32PM +0100, Michal Hocko wrote: > On Thu 23-03-23 07:52:22, Marcelo Tosatti wrote: > > On Thu, Mar 23, 2023 at 08:51:14AM +0100, Michal Hocko wrote: > > > On Wed 22-03-23 11:20:55, Marcelo Tosatti wrote: > > > > On Wed, Mar 22, 2023 at 02:35:20PM +0100, Michal Hocko wrote: > > > [...] > > > > > > "Performance details for the kworker interruption: > > > > > > > > > > > > oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000) > > > > > > oslat 1094.456971: workqueue_queue_work: ... function=vmstat_update ... > > > > > > oslat 1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ... > > > > > > kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ... > > > > > > > > > > > > The example above shows an additional 7us for the > > > > > > > > > > > > oslat -> kworker -> oslat > > > > > > > > > > > > switches. In the case of a virtualized CPU, and the vmstat_update > > > > > > interruption in the host (of a qemu-kvm vcpu), the latency penalty > > > > > > observed in the guest is higher than 50us, violating the acceptable > > > > > > latency threshold for certain applications." > > > > > > > > > > Yes, I have seen that but it doesn't really give a wider context to > > > > > understand why those numbers matter. > > > > > > > > OK. > > > > > > > > "In the case of RAN, a MAC scheduler with TTI=1ms, this causes >100us > > > > interruption observed in a guest (which is above the safety > > > > threshold for this application)." > > > > > > > > Is that OK? > > > > > > This might be a sufficient information for somebody familiar with the > > > matter (not me). So no, not enough. We need to hear a more complete > > > story. > > > > Michal, > > > > Please refer to > > https://www.diva-portal.org/smash/get/diva2:541460/FULLTEXT01.pdf > > > > 2.3 Channel Dependent Scheduling > > The purpose of scheduling is to decide which terminal will transmit data on which set > > of resource blocks with what transport format to use. The objective is to assign > > resources to the terminal such that the quality of service (QoS) requirement is fulfilled. > > Scheduling decision is taken every 1 ms by base station (termed as eNodeB) as the > > same length of Transmission Time Interval (TTI) in LTE system. > > > > In general: > > > > https://en.wikipedia.org/wiki/Real-time_computing > > Thank you, but not something I was really asking for (repeatedly). I am > pretty aware of what RT computing is about. I am not really interested > in a generic fluff. I am asking about specific usecases you have in mind > when pushing these changes. > > > For example, for the MAC scheduler processing must occur every 1ms, > > and a certain amount of computation takes place (and must finish before > > the next 1ms timeframe). A > 50us latency spike as observed by cyclictest > > is considered a "failure". > > OK, you are claiming that much but you are not really filling up other > holes in your story. Let me just outline few questions I have. Your > measurements talk about 7us overhead the vmstat processing might add. > This is really far from > 50us above. 7us in the host, for the following sched_switch events: oslat -> kworker kworker -> oslat However, if the impact is for a virtualized application: oslat, executing via qemu-vcpu process in the host. oslat executing qemu-vcpu VM-EXIT qemu-vcpu -> kworker kworker -> qemu-vcpu qemu-vcpu VM-ENTRY is much higher than the 7us (can be above 100us). > You suggest that this is an effect > of the workload running in a guest without more details. I am quite > surprised to hear about RT expectations inside a guest system TBH. https://www.youtube.com/watch?v=zIDwc6uDszY > All that being said, it would be really helpful if you were more > specific about the workload and why there is no other way but making > vmstat infrastructure more complex (it is quite complex on its own). The patchset is just changing vmstat_shepherd from happening locally to happening remotely. There are a number of algorithms in the kernel that deal with concurrent access already. What you think this particular patchset makes things complicated and what can be done to make it simpler?
