On Thu, Feb 24, 2022 at 09:27:14AM -0300, Marcelo Tosatti wrote: > On Sat, Feb 19, 2022 at 03:46:16PM +0000, Aaron Tomlin wrote: > > On Fri 2022-02-18 12:54 +0000, Aaron Tomlin wrote: > > > On Thu 2022-02-17 17:32 +0100, Frederic Weisbecker wrote: > > > > > If I understand correctly, in the context of nohz_full, since such work is > > > > > deferred, it will only be handled in a scenario when the periodic/or > > > > > scheduling-clock tick is enabled i.e. the timer was reprogrammed on exit > > > > > from idle. > > > > > > > > Oh I see, it's a deferrable delayed work... > > > > Then I can see two other issues: > > > > > > > > 1) Can an interrupt in idle modify the vmstat and thus trigger the need to > > > > flush it? > > Yes. Page allocation and page freeing for example. > > 6 3730 ../mm/page_alloc.c <<rmqueue>> > __mod_zone_freepage_state(zone, -(1 << order), > 4 1096 ../mm/page_alloc.c <<__free_one_page>> > __mod_zone_freepage_state(zone, -(1 << order), > > > > > I believe it's the case and then the problem goes beyond nohz_full > > > > because if the idle interrupt fired while the tick is stopped and didn't set > > > > TIF_RESCHED, we go back to sleep without calling quiet_vmstat(). > > > > > > Yes: e.g. a nohz_full CPU, in idle code, could indeed receive a reschedule > > > IPI; re-enable local IRQs and generic idle code sees the TIF_NEED_RESCHED > > > flag against the idle task. Additionally, the selected task could > > > indirectly released a few pages [to satisfy a low-memory condition] and > > > modify CPU-specific vmstat data i.e. vm_stat_diff[NR_FREE_PAGES]. > > > > > > > > > > 2) What if we are running task A in kernel mode while the tick is stopped > > > > (nohz_full). Task A modifies the vmstat and goes to userspace for a long > > > > while. > > > > Your patch fixes case 1) but not case 2). The problem is that TIMER_DEFERRABLE > > > > should really be about dynticks-idle only and not dynticks-full. I've always > > > > been afraid about enforcing that rule though because that would break old > > > > noise-free setups. But perhaps I should... Can't grasp the sentence above "The problem is that ...". What rule?
