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

Janosch Frank
2020-04-29T12:10:47Z

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On 4/29/20 1:47 PM, David Hildenbrand wrote:
> 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.
>

For now I'd rather have a workaround in the test until I can find cycles
to find a solution in KVM/QEMU.

SIGP SENSE has been working quite well for Linux for the last few years,
so I won't start running around now frantically fixing stuff.

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