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? > 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. [...] > mm/vmstat.c | 440 +++++++++++++++++++++++++++++++++++++++++++++++------------------------------ This requires much more detailed story why we really need that. -- Michal Hocko SUSE Labs
