On Tue, Dec 13, 2016 at 12:13:22PM +0100, Martin Schwidefsky wrote: > On Mon, 12 Dec 2016 16:02:30 +0100 > Frederic Weisbecker <[email protected]> wrote: > > > On Mon, Dec 12, 2016 at 11:27:54AM +0100, Martin Schwidefsky wrote: > > > 3) The call to vtime_flush in account_process_tick is done in irq context > > > from > > > update_process_times. hardirq_offset==1 is also correct. > > > > Let's see this for example: > > > > + if ((tsk->flags & PF_VCPU) && (irq_count() - hardirq_offset == 0)) > > + S390_lowcore.guest_timer += timer; > > > > If the tick is interrupting guest, we have accounted the guest time on tick > > IRQ entry. > > Now we are in the middle of the tick interrupt and since hardirq_offset is > > 1, we > > are taking the above path by accounting half of the tick-IRQ time as guest, > > which is wrong, > > it's actually IRQ time. > > Hmm, you got me there. The system time from irq_enter until > account_process_tick > is reached is indeed IRQ time. It is not much but it is incorrect. The best > fix > would be to rip out the accounting of the system time from > account_process_tick > as irq_enter / irq_exit will do system time accounting anyway. To do that > do_account_vtime needs to be split, because for the task switch we need to > account the system time of the previous task.
Exactly!
