On Thu 2022-02-17 17:32 +0100, Frederic Weisbecker wrote: > Then I can see two other issues: > > 1) Can an interrupt in idle modify the vmstat and thus trigger the need to > flush it? 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(). > > 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... Hi Frederic, Firstly, apologies for the delay. In reference to case 2: If I understand correctly, even if TIMER_DEFERRABLE is removed refresh_cpu_vm_stats() cannot be invoked since the scheduling-clock tick is disabled i.e. non-deferrable timers are serviced by the tick, no? So, the only option would be to interrupt the workload - not desirable - or detect any remaining differentials prior to entering userspace? Kind regards, -- Aaron Tomlin
