Re: [PATCH] time,virt: resync steal time when guest & host lose sync

Ingo Molnar
2016-08-13T08:43:07Z

* Rik van Riel <riel@redhat.com> wrote:

> On Wed, 10 Aug 2016 07:39:08 +0800
> Wanpeng Li <kernellwp@gmail.com> wrote:
>
> > The regression is caused by your commit "sched,time: Count actually
> > elapsed irq & softirq time".
>
> Wanpeng, does this patch fix your issue?
>
> Paolo, what is your opinion on this issue?
>
> I can think of all kinds of ways in which guest and host might lose
> sync with steal time, from uninitialized values at boot, to guest
> pause, followed by save to disk, and reload, to live migration, to...
>
> ---8<---
>
> Subject: time,virt: resync steal time when guest & host lose sync
>
> When guest and host wildly disagree on steal time, a guest can
> do several things:
> 1) Quickly account all the steal time at once (the kernel did this before
> 57430218317e ("sched/cputime: Count actually elapsed irq & softirq time"),
> when steal_account_process_ticks got ULONG_MAX as its maximum value.
> 2) Stay out of sync for an indeterminate amount of time. This is what the
> system does today.
> 3) Sync up the guest value to the host-provided value, without accounting
> an absurdly large value in the cpu time statistics.
>
> This patch makes the kernel do (3), which seems like the right thing
> to do.
>
> The exact value of the threshold use probably does not matter too much,
> as long as it is long enough to cover all the timer ticks that passed
> during an idle period, because (irqtime_)account_idle_ticks can process
> a large amount of time all at once.
>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> Reported-by: Wanpeng Li <kernellwp@gmail.com>
> ---
> kernel/sched/cputime.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)

fails to build on x86 allnoconfig:

kernel/sched/cputime.c:524:10: error: too many arguments to function ‘steal_account_process_time’

Thanks,

Ingo