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

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