Re: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.

[Dongli Zhang]

On 11/23/21 3:50 PM, Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
wrote:
> 
> 
>> -----Original Message-----
>> From: Dongli Zhang [mailto:dongli.zhang@xxxxxxxxxx]
>> Sent: Wednesday, November 24, 2021 5:22 AM
>> To: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>; Longpeng (Mike, Cloud
>> Infrastructure Service Product Dept.) <longpeng2@xxxxxxxxxx>
>> Cc: linux-kernel@xxxxxxxxxxxxxxx; Gonglei (Arei) <arei.gonglei@xxxxxxxxxx>;
>> x86@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; Peter Zijlstra
>> <peterz@xxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Valentin Schneider
>> <valentin.schneider@xxxxxxx>; Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>;
>> Juergen Gross <jgross@xxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>;
>> Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Borislav
>> Petkov <bp@xxxxxxxxx>; Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>; H. Peter
>> Anvin <hpa@xxxxxxxxx>
>> Subject: Re: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be
>> brought up again.
>>
>> Tested-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
>>
>>
>> The bug fixed by commit 53fafdbb8b21 ("KVM: x86: switch KVMCLOCK base to
>> monotonic raw clock") may leave the cpu_hotplug_state at CPU_UP_PREPARE. As a
>> result, to online this CPU again (even after removal) is always failed.
>>
>> I have tested that this patch works well to workaround the issue, by introducing
>> either a mdeley(11000) or while(1); to start_secondary(). That is, to online
>> the
>> same CPU again is successful even after initial do_boot_cpu() failure.
>>
>> 1. add mdelay(11000) or while(1); to the start_secondary().
>>
>> 2. to online CPU is failed at do_boot_cpu().
>>
> 
> Thanks for your testing :)
> 
> Does the cpu4 spin in wait_for_master_cpu() in your case ?

I did two tests.

TEST 1.

I added "mdelay(11000);" as the first line in start_secondary(). Once the issue
was encountered, the RIP of CPU=4 was ffffffff8c242021 (from QEMU's "info
registers -a") which was in the range of wait_for_master_cpu().

# cat /proc/kallsyms | grep ffffffff8c2420
ffffffff8c242010 t wait_for_master_cpu
ffffffff8c242030 T load_fixmap_gdt
ffffffff8c242060 T native_write_cr4
ffffffff8c2420c0 T cr4_init


TEST 2.

I added "while(true);" as the first line in start_secondary(). Once the issue
was encountered, the RIP of CPU=4 was ffffffff91654c0a (from QEMU's "info
registers -a") which was in the range of start_secondary().

# cat /proc/kallsyms | grep ffffffff91654c0
ffffffff91654c00 t start_secondary

Dongli Zhang


> 
>> 3. to online CPU again is failed without this patch.
>>
>> # echo 1 > /sys/devices/system/cpu/cpu4/online
>> -su: echo: write error: Input/output error
>>
>> 4. to online CPU again is successful with this patch.
>>
>> Thank you very much!
>>
>> Dongli Zhang
>>
>> On 11/22/21 7:47 AM, Sebastian Andrzej Siewior wrote:
>>> From: "Longpeng(Mike)" <longpeng2@xxxxxxxxxx>
>>>
>>> A CPU will not show up in virtualized environment which includes an
>>> Enclave. The VM splits its resources into a primary VM and a Enclave
>>> VM. While the Enclave is active, the hypervisor will ignore all requests
>>> to bring up a CPU and this CPU will remain in CPU_UP_PREPARE state.
>>> The kernel will wait up to ten seconds for CPU to show up
>>> (do_boot_cpu()) and then rollback the hotplug state back to
>>> CPUHP_OFFLINE leaving the CPU state in CPU_UP_PREPARE. The CPU state is
>>> set back to CPUHP_TEARDOWN_CPU during the CPU_POST_DEAD stage.
>>>
>>> After the Enclave VM terminates, the primary VM can bring up the CPU
>>> again.
>>>
>>> Allow to bring up the CPU if it is in the CPU_UP_PREPARE state.
>>>
>>> [bigeasy: Rewrite commit description.]
>>>
>>> Signed-off-by: Longpeng(Mike) <longpeng2@xxxxxxxxxx>
>>> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
>>> Link:
>> https://urldefense.com/v3/__https://lore.kernel.org/r/20210901051143.2752-1
>> -longpeng2@huawei.com__;!!ACWV5N9M2RV99hQ!d4sCCXMQV7ekFwpd21vo1_9K-m5h4VZ-g
>> E8Z62PLL58DT4VJ6StH57TR_KpBdbwhBE0$
>>> ---
>>>
>>> For XEN: this changes the behaviour as it allows to invoke
>>> cpu_initialize_context() again should it have have earlier. I *think*
>>> this is okay and would to bring up the CPU again should the memory
>>> allocation in cpu_initialize_context() fail.
>>>
>>>  kernel/smpboot.c | 7 +++++++
>>>  1 file changed, 7 insertions(+)
>>>
>>> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
>>> index f6bc0bc8a2aab..34958d7fe2c1c 100644
>>> --- a/kernel/smpboot.c
>>> +++ b/kernel/smpboot.c
>>> @@ -392,6 +392,13 @@ int cpu_check_up_prepare(int cpu)
>>>  		 */
>>>  		return -EAGAIN;
>>>
>>> +	case CPU_UP_PREPARE:
>>> +		/*
>>> +		 * Timeout while waiting for the CPU to show up. Allow to try
>>> +		 * again later.
>>> +		 */
>>> +		return 0;
>>> +
>>>  	default:
>>>
>>>  		/* Should not happen.  Famous last words. */
>>>