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Re: Trying add smt=off disabled cores to cpupool crash xen
- To: Juergen Gross <jgross@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 4 Dec 2023 11:13:35 +0100
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- Cc: George Dunlap <george.dunlap@xxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, René Winther Højgaard <renewin@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Delivery-date: Mon, 04 Dec 2023 10:13:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 04.12.2023 11:02, Juergen Gross wrote:
> On 04.12.23 10:15, Jan Beulich wrote:
>> On 01.12.2023 21:12, Andrew Cooper wrote:
>>> On 01/12/2023 7:59 pm, René Winther Højgaard wrote:
>>>> If I set smt=off and try to configure cpupools with credit(1) as if
>>>> all cores are available, I get the following crash.
>>>>
>>>> The crash happens when I try to use xl cpupool-add-cpu on the disabled
>>>> HT sibling cores.
>>>>
>>>> Hyper-threading is enabled in the firmware, and only disabled with
>>>> smt=off.
>>>
>>> CC'ing some maintainers.
>>>
>>> I expect this will also explode when a CPU is runtime offlined with
>>> `xen-hptool cpu-offline` and then added to a cpupool.
>>>
>>> Interestingly, the crash is mov (%rdx,%rax,1),%r13, and I think that's
>>> the percpu posion value in %rdx.
>>>
>>> I expect cpupools want to reject parked/offline CPUs.
>>
>> While the only explicit check there is
>>
>> if ( cpu >= nr_cpu_ids )
>> goto addcpu_out;
>>
>> I would have expected this
>>
>> if ( !cpumask_subset(cpus, &cpupool_free_cpus) ||
>> cpumask_intersects(cpus, &cpupool_locked_cpus) )
>> goto addcpu_out;
>>
>> to deal with the situation, as parked/offline CPUs shouldn't be "free".
>> Jürgen?
>
> The problem is the call of sched_get_opt_cpumask() to need the percpu area
> of the cpu in question.
That was my first thought, too, but then I saw cpupool_assign_cpu_locked() on
the call trace, which is called only afterwards. Plus sched_get_opt_cpumask()
needs the per-CPU area only when granularity was switched from its default of
SCHED_GRAN_cpu afaics.
Jan
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