On Wed 22-03-23 08:23:21, Marcelo Tosatti wrote: > On Wed, Mar 22, 2023 at 11:13:02AM +0100, Michal Hocko wrote: > > On Mon 20-03-23 16:07:29, Marcelo Tosatti wrote: > > > 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. > > > > Those workloads shouldn't enter the kernel in the first place, no? > > It depends on the latency requirements and individual system calls. > > > Otherwise the in kernel execution with all the direct or indirect > > dependencies (e.g. via locks) can throw any latency expectations off the > > window. > > > > > > > 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 > > > > Yes, I am not disputing low latency workloads in general. I am just > > saying that you haven't really established a very sound justification > > here. > > The -v7 cover letter was updated with additional details, > as you requested (perhaps you missed it): > > "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. > > Of course there are workloads which do not want to conflict with > > any in kernel house keeping. Those have to be configured and implemented > > very carefully though. Vmstat as such should not collide with those > > workloads as long as they do not interact with the kernel in a way > > counters are updated. Is this hard or impossible to avoid? > > The practical problem we have been seeing is -RT app initialization. > For example: > > 1) mlock(); > 2) enter loop without system calls OK, that is what I have kinda expected. Would have been better to mention it explicitly. I expect this to be a very common pattern and vmstat might not be the only subsystem that could interfere later on. Would it make more sense to address this by a more generic solution? E.g. a syscall to flush all per-cpu caches so they won't interfere later unless userspace hits the kernel path in some way (e.g. flush_cpu_caches(cpu_set_t cpumask, int flags)? The above pattern could then be implemented as do_initial_setup() sched_setaffinity(getpid(), cpumask); flush_cpu_caches(cpumask, 0); do_userspace_loop() -- Michal Hocko SUSE Labs
