Re: [PATCH] sched/cputime: add steal clock warps handling during cpu hotplug

[Wanpeng Li]

2016-06-02 21:59 GMT+08:00 Rik van Riel <riel@xxxxxxxxxx>:
> On Thu, 2016-06-02 at 14:00 +0200, Peter Zijlstra wrote:
>> On Thu, Jun 02, 2016 at 07:57:19PM +0800, Wanpeng Li wrote:
>> >
>> > From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> >
>> > I observed that sometimes st is 100% instantaneous, then idle is
>> > 100%
>> > even if there is a cpu hog on the guest cpu after the cpu hotplug
>> > comes
>> > back(N.B. both guest and host are latest 4.7-rc1, this can not
>> > always
>> > be readily reproduced). I add trace to capture it as below:
>> >
>> > cpuhp/1-12    [001] d.h1   167.461657: account_process_tick: steal
>> > = 1291385514, prev_steal_time = 0
>> > cpuhp/1-12    [001] d.h1   167.461659: account_process_tick:
>> > steal_jiffies = 1291
>> > <idle>-0     [001] d.h1   167.462663: account_process_tick: steal =
>> > 18732255, prev_steal_time = 1291000000
>> > <idle>-0     [001] d.h1   167.462664: account_process_tick:
>> > steal_jiffies = 18446744072437
>> >
>> > The steal clock warps and then steal_jiffies overflow, this patch
>> > align
>> > prev_steal_time to the new steal clock timestamp, in order to
>> > avoid
>> > overflow and st stuff can continue to work.
>> I would rather suggest fixing the steal clock thing to not jump like
>> that; is that at all possible?
>
> Not always possible, I suspect.
>
> If a guest is saved to disk and later restored (eg. after
> a host reboot), or live migrated to another host, I would
> expect to get totally disjoint steal time statistics from
> the "new run" of the guest (which is the same run of the
> guest OS).
>
> In fact, this code may also need to deal with the case
> where steal time suddenly increases by a ludicrous amount,
> and ignore those events, too.
>
> A safe threshold might be to only apply steal times that
> are positive and smaller than one second (as long as nohz_full
> has the one second timer tick left), ignoring intervals that
> are negative or longer than a second, and using those to sync
> up the guest with the host.

Good point, thanks for your review, Rik. :) Just send out v2 to do it.

Regards,
Wanpeng Li