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

Heiko Carstens
2021-08-03T08:42:51Z

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.