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

[Juergen Gross <jgross@suse.com>]

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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