Re: [PATCH 3/3] powerpc/smp: Cache CPU to chip lookup

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Gautham R Shenoy <ego@linux.vnet.ibm.com> [2021-04-15 22:49:21]:

> > 
> > +int *chip_id_lookup_table;
> > +
> >  #ifdef CONFIG_PPC64
> >  int __initdata iommu_is_off;
> >  int __initdata iommu_force_on;
> > @@ -914,13 +916,22 @@ EXPORT_SYMBOL(of_get_ibm_chip_id);
> >  int cpu_to_chip_id(int cpu)
> >  {
> >  	struct device_node *np;
> > +	int ret = -1, idx;
> > +
> > +	idx = cpu / threads_per_core;
> > +	if (chip_id_lookup_table && chip_id_lookup_table[idx] != -1)
> 

> The value -1 is ambiguous since we won't be able to determine if
> it is because we haven't yet made a of_get_ibm_chip_id() call
> or if of_get_ibm_chip_id() call was made and it returned a -1.
> 

We don't allocate chip_id_lookup_table unless cpu_to_chip_id() return
!-1 value for the boot-cpuid. So this ensures that we dont
unnecessarily allocate chip_id_lookup_table. Also I check for
chip_id_lookup_table before calling cpu_to_chip_id() for other CPUs.
So this avoids overhead of calling cpu_to_chip_id() for platforms that
dont support it.  Also its most likely that if the
chip_id_lookup_table is initialized then of_get_ibm_chip_id() call
would return a valid value.

+ Below we are only populating the lookup table, only when the
of_get_cpu_node is valid.

So I dont see any drawbacks of initializing it to -1. Do you see any?

> Thus, perhaps we can initialize chip_id_lookup_table[idx] with a
> different unique negative value. How about S32_MIN ? and check
> chip_id_lookup_table[idx] is different here ?
> 

I had initially initialized to -2, But then I thought we adding in
more confusion than necessary and it was not solving any issues.


-- 
Thanks and Regards
Srikar Dronamraju