Re: [PATCH 09/10] s390/cputime: delayed accounting of system time

Martin Schwidefsky
2016-12-20T14:14:05Z

On Wed, 14 Dec 2016 02:44:46 +0100
Frederic Weisbecker <fweisbec@gmail.com> wrote:

> On Tue, Dec 13, 2016 at 02:21:21PM +0100, Martin Schwidefsky wrote:
> > On Tue, 13 Dec 2016 12:13:22 +0100
> > Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
> >
> > > On Mon, 12 Dec 2016 16:02:30 +0100
> > > Frederic Weisbecker <fweisbec@gmail.com> 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.
> >
> > New patch for the delayed cputime account. I can not just rip out system time
> > accounting from account_process_tick after all, I need a sync point for the
> > steal time calculation. It basically is the same patch as before but with a new
> > helper update_tsk_timer, the removal of hardirq_offset and a simplification
> > for do_account_vtime: the last accounting delta is either hardirq time for
> > the tick or system time for the task switch.
> >
> > Keeping my finger crossed..
>
> The patch looks good. But you might want to remove the hardirq_offset in a
> separate patch to queue for this merge window (I'm not sure if it needs a
> stable tag, the argument may be there since the beginning).
>
> Because the rest depends on the series that is unlikely to be queued in this
> merge window at this stage.

I just pushed two fixes to the linux-s390:fixes tree which will be merged
eventually after the first -rc candidate for 4.10 is released.

This includes "s390/vtime: correct system time accounting" which fixes the
hardirq_offset bug for s390.

The link to the fixes-tree in case you want to look at the patch:

git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux.git fixes

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.