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Date:   Tue, 3 Aug 2021 10:42:37 +0200
From:   Heiko Carstens <hca@linux.ibm.com>
To:     Pierre Morel <pmorel@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,
        thuth@redhat.com, imbrenda@linux.ibm.com, gor@linux.ibm.com
Subject: Re: [PATCH v3 3/3] s390x: optimization of the check for CPU topology
 change
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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.