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Subject: Re: [PATCH v3 3/3] s390x: optimization of the check for CPU topology
 change
To:     Heiko Carstens <hca@linux.ibm.com>
Cc:     kvm@vger.kernel.org, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, borntraeger@de.ibm.com,
        frankja@linux.ibm.com, cohuck@redhat.com, david@redhat.com,
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References: <1627979206-32663-1-git-send-email-pmorel@linux.ibm.com>
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From:   Pierre Morel <pmorel@linux.ibm.com>
Message-ID: <b91ce49f-c73b-bdd2-2389-8313f4baf46c@linux.ibm.com>
Date:   Tue, 3 Aug 2021 10:57:15 +0200
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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@linux.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?


-- 
Pierre Morel
IBM Lab Boeblingen