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 with the goal to: "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 returning errors from access to /proc/sys/vm/stat_refresh. Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> --- v3: improve changelog (Michal Hocko) v2: opencode schedule_on_each_cpu (Michal Hocko) Index: linux-vmstat-remote/mm/vmstat.c =================================================================== --- linux-vmstat-remote.orig/mm/vmstat.c +++ linux-vmstat-remote/mm/vmstat.c @@ -1881,8 +1881,13 @@ int vmstat_refresh(struct ctl_table *tab void *buffer, size_t *lenp, loff_t *ppos) { long val; - int err; int i; + int cpu; + struct work_struct __percpu *works; + + works = alloc_percpu(struct work_struct); + if (!works) + return -ENOMEM; /* * The regular update, every sysctl_stat_interval, may come later @@ -1896,9 +1901,24 @@ int vmstat_refresh(struct ctl_table *tab * transiently negative values, report an error here if any of * the stats is negative, so we know to go looking for imbalance. */ - err = schedule_on_each_cpu(refresh_vm_stats); - if (err) - return err; + cpus_read_lock(); + for_each_online_cpu(cpu) { + struct work_struct *work; + + if (cpu_is_isolated(cpu)) + continue; + work = per_cpu_ptr(works, cpu); + INIT_WORK(work, refresh_vm_stats); + schedule_work_on(cpu, work); + } + + for_each_online_cpu(cpu) { + if (cpu_is_isolated(cpu)) + continue; + flush_work(per_cpu_ptr(works, cpu)); + } + cpus_read_unlock(); + free_percpu(works); for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) { /* * Skip checking stats known to go negative occasionally.
