Re: [PATCH v5 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug

[Wanpeng Li]

2016-06-08 18:01 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>
>
> On 08/06/2016 05:05, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>> Commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug")
>> set rq->prev_* to 0 after a cpu hotplug comes back in order to fix the scenario:
>>
>> | steal is smaller than rq->prev_steal_time we end up with an insane large
>> | value which then gets added to rq->prev_steal_time, resulting in a permanent
>> | wreckage of the accounting.
>>
>> However, it is still buggy.
>>
>> rq->prev_steal_time = 0:
>>
>> As Rik pointed out:
>>
>> | setting rq->prev_irq_time to 0 in the guest, and then getting a giant value from
>> | the host, could result in a very large of steal_jiffies.
>>
>> rq->prev_steal_time_rq = 0:
>>
>> | steal = paravirt_steal_clock(cpu_of(rq));
>> | steal -= rq->prev_steal_time_rq;
>> |
>> | if (unlikely(steal > delta))
>> |      steal = delta;
>> |
>> | rq->prev_steal_time_rq += steal;
>> | delta -= steal;
>> |
>> | rq->clock_task += delta;
>>
>> steal is a giant value and rq->prev_steal_time_rq is 0, rq->prev_steal_time_rq
>> grows in delta granularity, rq->clock_task can't ramp up until rq->prev_steal_time_rq
>> catches up steal clock since delta value will be 0 after reducing steal time from
>> normal execution time. That's why I obersved that cpuhg/1-12 continue running
>> until rq->prev_steal_time_rq catches up steal clock timestamp.
>>
>> I believe rq->prev_irq_time has similar issue. So this patch fix it by reverting
>> commit e9532e69b8d1.
>
> This is still very hard to read.  I think the justification for this
> patch is much simpler:
>
> -------
> Commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU
> hotplug") set rq->prev_* to 0 after a cpu hotplug comes back in order to
> fix the case where (after CPU hotplug) steal is smaller than
> rq->prev_steal_time.
>
> However, this should never happen. steal was only smaller because of the
> KVM-specific bug fixed by the previous patch.  Worse, the previous patch
> triggers a bug on CPU hot-unplug/plug operation: because
> rq->prev_steal_time is cleared, all of the CPU's past steal time will be
> accounted again on hot-plug.
>
> Since the root cause has been fixed, we can just revert commit e9532e69b8d1.
> -------

Thanks Paolo! :)

Regards,
Wanpeng Li