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.
-------
Since things are currently pretty broken, I think it's fair to keep the
two patches separate instead of squashing them.
Paolo
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