linux-kernel - Re: [PATCH v3 3/3] s390x: optimization of the check for CPU topology change

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Date:   Tue, 3 Aug 2021 11:28:56 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, borntraeger@...ibm.com,
        frankja@...ux.ibm.com, cohuck@...hat.com, david@...hat.com,
        thuth@...hat.com, imbrenda@...ux.ibm.com, gor@...ux.ibm.com
Subject: Re: [PATCH v3 3/3] s390x: optimization of the check for CPU topology
 change



On 8/3/21 10:57 AM, Pierre Morel wrote:
> 
> 
> On 8/3/21 10:42 AM, Heiko Carstens wrote:
>> On Tue, Aug 03, 2021 at 10:26:46AM +0200, Pierre Morel wrote:
>>> Now that the PTF instruction is interpreted by the SIE we can optimize
>>> the arch_update_cpu_topology callback to check if there is a real need
>>> to update the topology by using the PTF instruction.
>>>
>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>> ---
>>>   arch/s390/kernel/topology.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/arch/s390/kernel/topology.c b/arch/s390/kernel/topology.c
>>> index 26aa2614ee35..741cb447e78e 100644
>>> --- a/arch/s390/kernel/topology.c
>>> +++ b/arch/s390/kernel/topology.c
>>> @@ -322,6 +322,9 @@ int arch_update_cpu_topology(void)
>>>       struct device *dev;
>>>       int cpu, rc;
>>> +    if (!ptf(PTF_CHECK))
>>> +        return 0;
>>> +
>>
>> We have a timer which checks if topology changed and then triggers a
>> call to arch_update_cpu_topology() via rebuild_sched_domains().
>> With this change topology changes would get lost.
> 
> For my understanding, if PTF check return 0 it means that there are no 
> topology changes.
> So they could not get lost.
> 
> What did I miss?
> 
> 
I missed that PTF clears the MCTR... and only one of the two calls will 
return 1 while we need both to return 1...


-- 
Pierre Morel
IBM Lab Boeblingen