Re: [PATCH] arm64: topology: Avoid checking numa mask for scheduler MC selection

[Jeremy Linton]

On 06/06/2018 05:05 AM, Sudeep Holla wrote:
> On 05/06/18 20:08, Jeremy Linton wrote:
> > The numa mask subset check has problems if !CONFIG_NUMA, over hotplug
> > operations or during early boot. Lets disable the NUMA siblings checks
> > for the time being, as NUMA in socket machines have LLC's that will
> > assure that the scheduler topology isn't "borken".
>
> ^ broken ? (not sure if usage of borken is intentional :))

Well that is what the scheduler says when it doesn't like the topology.

> > Futher, as a defensive mechanism during hotplug, lets assure that the
>
> ^ Further

Sure.

> > LLC siblings are also masked.
>
> Also add the symptoms of the issue we say as Geert suggested me.
> Something like:
> " This often leads to system hang or crash during CPU hotplug and system
> suspend operation. This is mostly observed on HMP systems where the
> CPU compute capacities are different and ends up in different scheduler
> domains. Since cpumask_of_node is returned instead core_sibling, the
> scheduler is confused with incorrect cpumasks(e.g. one CPU in two
> different sched domains at the same time) on CPU hotplug."
>
> You can add Reported-by: Geert... ?

ok.

> > Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> > ---
> >   arch/arm64/kernel/topology.c | 11 ++++-------
> >   1 file changed, 4 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> > index 7415c166281f..f845a8617812 100644
> > --- a/arch/arm64/kernel/topology.c
> > +++ b/arch/arm64/kernel/topology.c
> > @@ -215,13 +215,8 @@ EXPORT_SYMBOL_GPL(cpu_topology);
> >   
> >   const struct cpumask *cpu_coregroup_mask(int cpu)
> >   {
> > -	const cpumask_t *core_mask = cpumask_of_node(cpu_to_node(cpu));
> > +	const cpumask_t *core_mask = &cpu_topology[cpu].core_sibling;
> >   
> > -	/* Find the smaller of NUMA, core or LLC siblings */
> > -	if (cpumask_subset(&cpu_topology[cpu].core_sibling, core_mask)) {
> > -		/* not numa in package, lets use the package siblings */
> > -		core_mask = &cpu_topology[cpu].core_sibling;
> > -	}
> >   	if (cpu_topology[cpu].llc_id != -1) {
> >   		if (cpumask_subset(&cpu_topology[cpu].llc_siblings, core_mask))
> >   			core_mask = &cpu_topology[cpu].llc_siblings;
> > @@ -239,8 +234,10 @@ static void update_siblings_masks(unsigned int cpuid)
> >   	for_each_possible_cpu(cpu) {
> >   		cpu_topo = &cpu_topology[cpu];
> >   
> > -		if (cpuid_topo->llc_id == cpu_topo->llc_id)
> > +		if (cpuid_topo->llc_id == cpu_topo->llc_id) {
> >   			cpumask_set_cpu(cpu, &cpuid_topo->llc_siblings);
> > +			cpumask_set_cpu(cpuid, &cpu_topo->llc_siblings);
> > +		}
> >   
> >   		if (cpuid_topo->package_id != cpu_topo->package_id)
> >   			continue;
>
> Looks good to me for now. I might need to tweek it a bit when I add the
> support to update topology on hotplug. But that's for latter. For now,
>
> Reviewed-by: Sudeep Holla <sudeep.holla@xxxxxxx>