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

[Pierre Morel]

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@xxxxxxxxxxxxx>
> > ---
> >   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