On Mon, Mar 20, 2023 at 07:25:55PM +0100, Michal Hocko wrote: > On Mon 20-03-23 15:03:32, Marcelo Tosatti wrote: > > This patch series addresses the following two problems: > > > > 1. A customer provided evidence indicating that a process > > was stalled in direct reclaim: > > > This is addressed by the trivial patch 1. > > [...] > > 2. With a task that busy loops on a given CPU, > > the kworker interruption to execute vmstat_update > > is undesired and may exceed latency thresholds > > for certain applications. > > Yes it can but why does that matter? It matters for the application that is executing and expects not to be interrupted. > > By having vmstat_shepherd flush the per-CPU counters to the > > global counters from remote CPUs. > > > > This is done using cmpxchg to manipulate the counters, > > both CPU locally (via the account functions), > > and remotely (via cpu_vm_stats_fold). > > > > Thanks to Aaron Tomlin for diagnosing issue 1 and writing > > the initial patch series. > > > > > > 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. > > I do not think we have ever promissed any specific latency guarantees > for vmstat. These are statistics have been mostly used for debugging > purposes AFAIK. I am not aware of any specific user space use case that > would be latency sensitive. Your changelog doesn't go into details there > either. There is a class of workloads for which response time can be of interest. MAC scheduler is an example: https://par.nsf.gov/servlets/purl/10090368 Thanks!
