Re: The irq Affinity is changed after the patch(Fixes: b1a5a73e64e9 ("genirq/affinity: Spread vectors on node according to nr_cpu ratio"))

["chenxiang (M)" <chenxiang66@hisilicon.com>]

在 2019/12/8 15:42, George Spelvin 写道:
> On Tue, 19 Nov 2019 at 11:05:55 +0800, chenxiang (M)" <chenxiang66@hisilicon.com> wrote:
>> Sorry, I can't access the link you provide, but I can provide those
>> irqs' affinity in the attachment.  I also write a small testcase,
>> and find id is 1, 2, 3, 0 after calling sort().
>>
>> struct ex_s {
>>      int id;
>>      int cpus;
>> };
>> struct ex_s ex_total[4] = {
>>      {0, 8},
>>      {1, 8},
>>      {2, 8},
>>      {3, 8}
>> };
>>
>> static int cmp_func(const void *l, const void *r)
>> {
>>      const struct ex_s *ln = l;
>>      const struct ex_s *rn = r;
>>
>>      return ln->cpus - rn->cpus;
>> }
> Your cmp_func is the problem.  sort() in the Linux kernel, like
> qsort() in the C library, has never been stable, meaning that if
> cmp_func() returns 0, there is no guarantee what order *l and *r
> will end up in.  Minor changes to the implementation can change the
> result.  You're sorting on the cpus field, which is all 8, so your
> cmp_func is saying "I don't care what order the results appear in".
>
> (A web search on "stable sort" will produce decades of discussion
> on the subject, but basically an unstable sort has numerous
> implementation advantages, and problems can usually be solved by
> adjusting the comparison function.)
>
> If you want to sort by ->id if ->cpus is the same, then change the
> cmp_func to say so:
>
> static int cmp_func(const void *l, const void *r)
> {
>      const struct ex_s *ln = l;
>      const struct ex_s *rn = r;
>
>      if (ln->cpus != rn->cpus)
> 	return ln->cpus - rn->cpus;
>      else
> 	return ln->id - rn->id;
> }
> (This simple subtract-based compare depends on the fact that "cpus"
> and "id" are both guaranteed to fit into 31 bits.  If there's any chance
> The true difference could exceed INT_MAX, you need to get a bit fancier.)
>
> .

Thank you for your reply,  George.
In function ncpus_cmp_func(), it only sorts by ->ncpus (which makes the 
affinity a little odd if the cpus of all the nodes are the same though 
it doesn't affect the function).
Maybe it is more better to sort by ->id if ->ncpus is the same in the 
function.