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

[Pierre Morel]

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@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?
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