[BUG] Paravirtual time accounting / IRQ time accounting

[lwcheng]

In consolidated environments, when there are multiple virtual machines (VMs)
running on one CPU core, timekeeping will be a problem to the guest OS.
Here, I report my findings about Linux process scheduler.


Description
------------
Linux CFS relies on rq->clock_task to charge each task, determine  
vruntime, etc.

When CONFIG_IRQ_TIME_ACCOUNTING is enabled, the time spent on serving IRQ
will be excluded from updating rq->clock_task.
When CONFIG_PARAVIRT_TIME_ACCOUNTING is enabled, the time stolen by  
the hypervisor
will also be excluded from updating rq->clock_task.

With "both" CONFIG_IRQ_TIME_ACCOUNTING and  
CONFIG_PARAVIRT_TIME_ACCOUNTING enabled,
I put three KVM guests on one core and run hackbench in each guest. I  
find that
in the guests, rq->clock_task stays *unchanged*. The malfunction  
embarrasses CFS.
------------


Analysis
------------
[src/kernel/sched/core.c]
static void update_rq_clock_task(struct rq *rq, s64 delta)
{
    ... ...
#ifdef CONFIG_IRQ_TIME_ACCOUNTING
    irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;
    ... ...
    rq->prev_irq_time += irq_delta;
    delta -= irq_delta;
#endif

#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
    if (static_key_false((&paravirt_steal_rq_enabled))) {
        steal = paravirt_steal_clock(cpu_of(rq));
        steal -= rq->prev_steal_time_rq;
        ... ...
        rq->prev_steal_time_rq += steal;
        delta -= steal;
    }
#endif

    rq->clock_task += delta;
    ... ...
}
--
"delta" -> the intended increment to rq->clock_task
"irq_delta" -> the time spent on serving IRQ (hard + soft)
"steal" -> the time stolen by the underlying hypervisor
--
"irq_delta" is calculated based on sched_clock_cpu(), which is vulnerable
to VM scheduling delays. "irq_delta" can include part or whole of "steal".
I observe that [irq_delta + steal >> delta].
As a result, "delta" becomes zero. That is why rq->clock_task stops.
------------

Please confirm this bug. Thanks.


Luwei Cheng
--
CS student
The University of Hong Kong
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