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

[Wanpeng Li <kernellwp@gmail.com>]

2016-08-16 22:01 GMT+08:00 Rik van Riel <riel@redhat.com>:
> On Tue, 2016-08-16 at 14:54 +0800, Wanpeng Li wrote:
>> 2016-08-16 10:11 GMT+08:00 Rik van Riel <riel@redhat.com>:
>> > On Tue, 2016-08-16 at 09:31 +0800, Wanpeng Li wrote:
>> > > 2016-08-15 23:00 GMT+08:00 Rik van Riel <riel@redhat.com>:
>> > > > On Mon, 2016-08-15 at 16:53 +0800, Wanpeng Li wrote:
>> > > > > 2016-08-12 23:58 GMT+08:00 Rik van Riel <riel@redhat.com>:
>> > > > > [...]
>> > > > > > Wanpeng, does the patch below work for you?
>> > > > >
>> > > > > It will break steal time for full dynticks guest, and there
>> > > > > is a
>> > > > > calltrace of thread_group_cputime_adjusted call stack, RIP is
>> > > > > cputime_adjust+0xff/0x130.
>> > > >
>> > > > How?  This patch is equivalent to passing ULONG_MAX to
>> > > > steal_account_process_time, which you tried to no ill
>> > > > effect before.
>> > >
>> > > https://lkml.org/lkml/2016/6/8/404/ Paolo original suggested to
>> > > add
>> > > the max cputime limit to the vtime, when the cpu is running in
>> > > nohz
>> > > full mode and stop the tick, jiffies will be updated depends on
>> > > clock
>> > > source instead of clock event device in
>> > > guest(tick_nohz_update_jiffies() callsite, ktime_get()), so it
>> > > will
>> > > not be affected by lost clock ticks, my patch keeps the limit for
>> > > vtime and remove the limit to non-vtime. However, your patch
>> > > removes
>> > > the limit for both scenarios and results in the below calltrace
>> > > for
>> > > vtime.
>> >
>> > I understand what it does.
>> >
>> > What I would like to understand is WHY enforcing the limit
>> > is the right thing when using vtime, and the wrong thing
>> > in all other scenarios.
>>
>> I observed that function get_vtime_delta() underflow which means that
>> delta < other when debugging your bugfix patch, I believe that is why
>> Paolo suggested to add the max cputime limit to vtime, he also
>> pointed
>> out the potentional underflow before
>> https://lkml.org/lkml/2016/6/8/404/
>
> Looking at get_vtime_delta() I can see exactly how the underflow
> can happen.  The interval returned by account_other_time() is NOT
> rounded down to the nearest jiffy, while the base interval it is
> subtracted from is.
>
> Furthermore, even if we did not have that rounding issue, a guest
> could get preempted in-between determining delta, and calling
> account_other_time(), which could also cause the issue.
>
> Could you re-send your patch with a comment in get_vtime_delta(),
> as well as the changelog, explaining exactly why account_other_time()
> should be limited from get_vtime_delta(), but not from the other
> three call sites?
>
> Documentation could save future developers a bunch of debugging
> time on this code.

Will do. Thanks for bearing with me through such a long discussion,
I'm very happy we finally come to an agreement. :)

Regards,
Wanpeng Li