Re: Another (ESP?) scsi blk-mq problem on sparc64

[Jens Axboe <axboe@kernel.dk>]

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On 11/24/2014 09:22 AM, Paul E. McKenney wrote:
> On Mon, Nov 24, 2014 at 08:35:46AM -0700, Jens Axboe wrote:
>> On 11/24/2014 01:21 AM, Christoph Hellwig wrote:
>>> On Fri, Nov 21, 2014 at 02:56:00PM -0500, David Miller wrote:
>>>> I would suggest looking into the possibility that we allocate the memory
>>>> using the count of valid cpus, rather than the largest cpu number.
>>>>
>>>> That's a common error that runs into problems with discontiguous
>>>> cpu numbering like Sparc sometimes has.
>>>
>>> Yes, that does look like the case.  Do you have a good trick on how
>>> to allocate a map for the highest possible cpu number without first
>>> iterating the cpu map?  I couldn't find something that looks like a
>>> highest_possible_cpu() helper.
>>
>> Honestly I think that num_posible_cpus() should return the max of
>> number of CPUs (weigt), and the highest numbered CPU. It's a pain in
>> the butt to handle this otherwise.
>
> Hear, hear!!!  That would make my life easier, and would make this sort
> of problem much less likely to occur!

How about this one?

-- 
Jens Axboe


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diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 0a9a6da21e74..db21f68aaef0 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -83,7 +83,13 @@ extern const struct cpumask *const cpu_active_mask;
 
 #if NR_CPUS > 1
 #define num_online_cpus()	cpumask_weight(cpu_online_mask)
-#define num_possible_cpus()	cpumask_weight(cpu_possible_mask)
+/*
+ * For platforms with discontig CPU numbering, the weight of the possible
+ * bitmask may be less than the highest numbered CPU. Return the max of
+ * the two, to prevent accidentical bugs.
+ */
+#define num_possible_cpus()	\
+	max_t(unsigned int, cpumask_weight(cpu_possible_mask), nr_cpu_ids)
 #define num_present_cpus()	cpumask_weight(cpu_present_mask)
 #define num_active_cpus()	cpumask_weight(cpu_active_mask)
 #define cpu_online(cpu)		cpumask_test_cpu((cpu), cpu_online_mask)