Re: [PATCH v2 08/10] s390x: smp: Wait for sigp completion

David Hildenbrand
2020-04-29T11:47:21Z

On 29.04.20 13:21, Janosch Frank wrote:
> On 4/29/20 11:55 AM, David Hildenbrand wrote:
>> On 29.04.20 11:37, Janosch Frank wrote:
>>> On 4/29/20 11:06 AM, David Hildenbrand wrote:
>>>> On 29.04.20 10:57, Janosch Frank wrote:
>>>>> On 4/24/20 1:40 PM, Janosch Frank wrote:
>>>>>> On 4/24/20 12:11 PM, David Hildenbrand wrote:
>>>>>>> On 23.04.20 11:10, Janosch Frank wrote:
>>>>>>>> Sigp orders are not necessarily finished when the processor finished
>>>>>>>> the sigp instruction. We need to poll if the order has been finished
>>>>>>>> before we continue.
>>>>>>>>
>>>>>>>> For (re)start and stop we already use sigp sense running and sigp
>>>>>>>> sense loops. But we still lack completion checks for stop and store
>>>>>>>> status, as well as the cpu resets.
>>>>>>>>
>>>>>>>> Let's add them.
>>>>>>>>
>>>>>>>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
>>>>>>>> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
>>>>>>>> ---
>>>>>>>> lib/s390x/smp.c | 8 ++++++++
>>>>>>>> lib/s390x/smp.h | 1 +
>>>>>>>> s390x/smp.c | 4 ++++
>>>>>>>> 3 files changed, 13 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/lib/s390x/smp.c b/lib/s390x/smp.c
>>>>>>>> index 6ef0335..2555bf4 100644
>>>>>>>> --- a/lib/s390x/smp.c
>>>>>>>> +++ b/lib/s390x/smp.c
>>>>>>>> @@ -154,6 +154,14 @@ int smp_cpu_start(uint16_t addr, struct psw psw)
>>>>>>>> return rc;
>>>>>>>> }
>>>>>>>>
>>>>>>>> +void smp_cpu_wait_for_completion(uint16_t addr)
>>>>>>>> +{
>>>>>>>> + uint32_t status;
>>>>>>>> +
>>>>>>>> + /* Loops when cc == 2, i.e. when the cpu is busy with a sigp order */
>>>>>>>> + sigp_retry(1, SIGP_SENSE, 0, &status);
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> int smp_cpu_destroy(uint16_t addr)
>>>>>>>> {
>>>>>>>> struct cpu *cpu;
>>>>>>>> diff --git a/lib/s390x/smp.h b/lib/s390x/smp.h
>>>>>>>> index ce63a89..a8b98c0 100644
>>>>>>>> --- a/lib/s390x/smp.h
>>>>>>>> +++ b/lib/s390x/smp.h
>>>>>>>> @@ -45,6 +45,7 @@ int smp_cpu_restart(uint16_t addr);
>>>>>>>> int smp_cpu_start(uint16_t addr, struct psw psw);
>>>>>>>> int smp_cpu_stop(uint16_t addr);
>>>>>>>> int smp_cpu_stop_store_status(uint16_t addr);
>>>>>>>> +void smp_cpu_wait_for_completion(uint16_t addr);
>>>>>>>> int smp_cpu_destroy(uint16_t addr);
>>>>>>>> int smp_cpu_setup(uint16_t addr, struct psw psw);
>>>>>>>> void smp_teardown(void);
>>>>>>>> diff --git a/s390x/smp.c b/s390x/smp.c
>>>>>>>> index 7462211..48321f4 100644
>>>>>>>> --- a/s390x/smp.c
>>>>>>>> +++ b/s390x/smp.c
>>>>>>>> @@ -75,6 +75,7 @@ static void test_stop_store_status(void)
>>>>>>>> lc->prefix_sa = 0;
>>>>>>>> lc->grs_sa[15] = 0;
>>>>>>>> smp_cpu_stop_store_status(1);
>>>>>>>> + smp_cpu_wait_for_completion(1);
>>>>>>>> mb();
>>>>>>>> report(lc->prefix_sa == (uint32_t)(uintptr_t)cpu->lowcore, "prefix");
>>>>>>>> report(lc->grs_sa[15], "stack");
>>>>>>>> @@ -85,6 +86,7 @@ static void test_stop_store_status(void)
>>>>>>>> lc->prefix_sa = 0;
>>>>>>>> lc->grs_sa[15] = 0;
>>>>>>>> smp_cpu_stop_store_status(1);
>>>>>>>
>>>>>>> Just curious: Would it make sense to add that inside
>>>>>>> smp_cpu_stop_store_status() instead?
>>>>>>>
>>>>>>
>>>>>> I think so, we also wait for stop and start to finish, so why not for
>>>>>> this order code.
>>>>>>
>>>>>
>>>>> I've moved the waiting into the smp library and now the prefix check for
>>>>> stop and store status fails every so often if executed repeatedly.
>>>>>
>>>>> I've tried making the lc ptr volatile, a print of the prefix before the
>>>>> report seems to fix the issue, a print after the report still shows the
>>>>> issue but according to the print both values are the same.
>>>>>
>>>>> I'm currently at a loss...
>>>>
>>>> Are you missing a barrier() somewhere?
>>>>
>>>
>>> Maybe, but the question is where?
>>>
>>> There's already one before the report:
>>> smp_cpu_stop_store_status(1);
>>> mb();
>>> report(lc->prefix_sa == (uint32_t)(uintptr_t)cpu->lowcore, "prefix");
>>
>> The issue here is:
>>
>> SIGP_SENSE is always handled in the kernel for KVM. Meaning, it will
>> complete even before the target CPU executed the stop and store (in QEMU).
>>
>> Reading the PoP:
>>
>> "One of the following conditions exists at the
>> addressed CPU: ... A previously issued stop-
>> and-store-status ... has been accepted by the
>> addressed CPU, and execution of the func-
>> tion requested by the order has not yet been
>> completed.
>>
>> "If the currently specified order is sense ... then the order
>> is rejected, and condition code 2 is set."
>>
>> So, in case of KVM, SENSE does not wait for completion of the previous
>> order. I remember that was a performance improvements, because we wanted
>> to avoid going to user space just to sense if another CPU is running.
>> (and I remember that the documentation was inconsistent)
>
> So, KVM is not architectural compliant when it comes to SIGP SENSE?
> I guess I need to go back to looping until the prefix is > 0

Yeah, or fix SIGP_SENSE in KVM. Would need QEMU and KVM changes. I
remember that a tricky part was checking if external calls are pending
for a CPU from user space.

We could pass that information along with the intercept to QEMU.

AFAIKs, SIGP SENSE is not used on a hot path in Linux.

--
Thanks,

David / dhildenb