On 2018-04-11 16:42:21 [+0200], To Tejun Heo wrote: > > > So is this perhaps related to the cpu hotplug that  mentions? e.g. is > > > the cpu being hotplugged cpu 1, the worker started too early before > > > stuff can be scheduled on the CPU, so it has to run on different than > > > designated CPU? > > > > > >  https://marc.info/?l=linux-mm&m=152088260625433&w=2 > > > > The report says that it happens when hotplug is attempted. Per-cpu > > doesn't pin the cpu alive, so if the cpu goes down while a work item > > is in flight or a work item is queued while a cpu is offline it'll end > > up executing on some other cpu. So, if a piece of code doesn't want > > that happening, it gotta interlock itself - ie. start queueing when > > the cpu comes online and flush and prevent further queueing when its > > cpu goes down. > > I missed that cpuhotplug part while reading it. So in that case, let me > add a CPU-hotplug notifier which cancels that work. After all it is not > need once the CPU is gone.
This already happens: - vmstat_shepherd() does get_online_cpus() and within this block it does queue_delayed_work_on(). So this has to wait until cpuhotplug completed before it can schedule something and then it won't schedule anything on the "off" CPU. - The work item itself (vmstat_update()) schedules itself (conditionally) again. - vmstat_cpu_down_prep() is the down event and does cancel_delayed_work_sync(). So it waits for the work-item to complete and cancels it. This looks all good to me. > > Thanks. Sebastian