Re: [Xen-devel] [PATCH v3 2/4] xen/sched: remove cpu from pool0 before removing it

[Juergen Gross <jgross@xxxxxxxx>]

On 13.09.19 19:27, Dario Faggioli wrote:
> On Mon, 2019-09-09 at 11:33 +0200, Juergen Gross wrote:
> > Today a cpu which is removed from the system is taken directly from
> > Pool0 to the offline state. This will conflict with the new idle
> > scheduler, so remove it from Pool0 first. Additionally accept
> > removing
> > a free cpu instead of requiring it to be in Pool0.
> >
> > For the resume failed case we need to call the scheduler code for
> > that
> > situation after the cpupool handling, so move the scheduler code into
> > a function and call it from cpupool_cpu_remove_forced() and remove
> > the
> > CPU_RESUME_FAILED case from cpu_schedule_callback().
> >
> > Note that we are calling now schedule_cpu_switch() in stop_machine
> > context so we need to switch from spinlock_irq to spinlock_irqsave.
> So, I was looking at this patch, and while doing that, also trying it
> out.
>
> I've done the following:
>
> # echo 0 > /sys/devices/system/xen_cpu/xen_cpu7/online
>
> And CPU 7 went offline, and was listed among the free CPUs:
>
> (XEN) Online Cpus: 0-6
> (XEN) Free Cpus: 7
> (XEN) Cpupool 0:
> (XEN) Cpus: 0-6
> (XEN) Scheduler: SMP Credit Scheduler rev2 (credit2)
> (XEN) Active queues: 1
> (XEN)   default-weight     = 256
> (XEN) Runqueue 0:
> (XEN)   ncpus              = 7
> (XEN)   cpus               = 0-6
> (XEN)   max_weight         = 256
> (XEN)   pick_bias          = 1
> (XEN)   instload           = 1
> (XEN)   aveload            = 3992 (~1%)
> (XEN)   idlers: 0000006f
> (XEN)   tickled: 00000000
> (XEN)   fully idle cores: 0000004f
>
> Then, I did:
>
> # echo 1 > /sys/devices/system/xen_cpu/xen_cpu7/online
>
> And again it appear to have worked, i.e., the CPU is back online and in
> Pool-0:
>
> (XEN) Online Cpus: 0-7
> (XEN) Cpupool 0:
> (XEN) Cpus: 0-7
> (XEN) Scheduler: SMP Credit Scheduler rev2 (credit2)
> (XEN) Active queues: 1
> (XEN)   default-weight     = 256
> (XEN) Runqueue 0:
> (XEN)   ncpus              = 8
> (XEN)   cpus               = 0-7
> (XEN)   max_weight         = 256
> (XEN)   pick_bias          = 1
> (XEN)   instload           = 2
> (XEN)   aveload            = 271474 (~103%)
> (XEN)   idlers: 000000af
> (XEN)   tickled: 00000000
> (XEN)   fully idle cores: 0000008f
>
> Then I did:
>
> # echo 0 > /sys/devices/system/xen_cpu/xen_cpu7/online
>
> And, after that:
>
> # xl cpupool-cpu-remove Pool-0 7
>
> And the system hanged.
>
> I don't have a working serial console on that testbox, unfortunately,
> so I can't poke at debug keys, etc.
>
> Is this anything that you've seen or that you can reproduce?
I can reproduce it and already have found the bug.


Juergen


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