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

[Gautham R Shenoy]

On Thu, Apr 15, 2021 at 11:21:10PM +0530, Srikar Dronamraju wrote:
> * 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?


Only if other callers of cpu_to_chip_id() don't check for whether the
chip_id_lookup_table() has been allocated or not. From a code
readability point of view, it is easier to have that check  this inside
cpu_to_chip_id() instead of requiring all its callers to make that
check.

> 
> > 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