On Mon, Jun 05, 2023 at 09:59:57AM +0200, Michal Hocko wrote: > On Fri 02-06-23 15:58:00, Marcelo Tosatti wrote: > > The interruption caused by queueing work on nohz_full CPUs > > is undesirable for certain aplications. > > This is not a proper changelog. I am not going to write a changelog for > you this time. Please explain why this is really needed and why this > approach is desired. > E.g. why don't you prevent userspace from > refreshing stats if interference is not desirable. Michal, Can you please check if the following looks better, as a changelog? thanks --- schedule_work_on API uses the workqueue mechanism to queue a work item on a queue. A kernel thread, which runs on the target CPU, executes those work items. Therefore, when using the schedule_work_on API, it is necessary for the kworker kernel thread to be scheduled in, for the work function to be executed. Time sensitive applications such as SoftPLCs (https://tum-esi.github.io/publications-list/PDF/2022-ETFA-How_Real_Time_Are_Virtual_PLCs.pdf), have their response times affected by such interruptions. The /proc/sys/vm/stat_refresh file was originally introduced by commit 52b6f46bc163eef17ecba4cd552beeafe2b24453 Author: Hugh Dickins <hughd@xxxxxxxxxx> Date: Thu May 19 17:12:50 2016 -0700 mm: /proc/sys/vm/stat_refresh to force vmstat update Provide /proc/sys/vm/stat_refresh to force an immediate update of per-cpu into global vmstats: useful to avoid a sleep(2) or whatever before checking counts when testing. Originally added to work around a bug which left counts stranded indefinitely on a cpu going idle (an inaccuracy magnified when small below-batch numbers represent "huge" amounts of memory), but I believe that bug is now fixed: nonetheless, this is still a useful knob. Other than the potential interruption to a time sensitive application, if using SCHED_FIFO or SCHED_RR priority on the isolated CPU, then system hangs can occur: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=978688 To avoid the problems above, do not schedule the work to synchronize per-CPU mm counters on isolated CPUs. Given the possibility for breaking existing userspace applications, avoid changing behaviour of access to /proc/sys/vm/stat_refresh, such as returning errors to userspace. --- > Also would it make some sense to reduce flushing to cpumask > of the calling process? (certainly a daring thought but have > you even considered it?) Fail to see the point here ?
