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

On Fri, Apr 16, 2021 at 09:27:48PM +0530, Gautham R Shenoy wrote:

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

quoted

* Gautham R Shenoy [off-list ref] [2021-04-15 22:49:21]:

quoted

quoted

+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)

quoted

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.

Even if they do, and the bad invalid value should never be read, I
think it's worth initializing that way.  If means if there's a mistake
and we do accidentally read the value, then the error is likely to be
much clearer.  Likewise if someone looks at this value from a
debugger, it will be clearer what's going on.

quoted

quoted

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.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson