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

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Date:   Tue, 20 Dec 2016 15:13:53 +0100
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Frederic Weisbecker <fweisbec@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Tony Luck <tony.luck@...el.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Mackerras <paulus@...ba.org>,
        Ingo Molnar <mingo@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Rik van Riel <riel@...hat.com>,
        Stanislaw Gruszka <sgruszka@...hat.com>
Subject: Re: [PATCH 09/10] s390/cputime: delayed accounting of system time

On Wed, 14 Dec 2016 02:44:46 +0100
Frederic Weisbecker <fweisbec@...il.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@...ibm.com> wrote:
> >   
> > > On Mon, 12 Dec 2016 16:02:30 +0100
> > > Frederic Weisbecker <fweisbec@...il.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.