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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Nov 24, 2014 at 10:16:15AM -0700, Jens Axboe wrote:
> 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?

Works for me!

(Just for the record, as far as I know, this doesn't matter for RCU,
which already uses nr_cpu_ids.)

							Thanx, Paul

> -- 
> Jens Axboe
> 

> 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)