Re: [PATCH 2/6] xen/sched: Use %*pb[l] instead of cpumask_scn{, list}printf()

[George Dunlap <george.dunlap@citrix.com>]

On 09/07/2018 04:17 PM, Jan Beulich wrote:
>>>> On 07.09.18 at 15:56, <andrew.cooper3@citrix.com> wrote:
>> On 07/09/18 09:03, Jan Beulich wrote:
>>>>>> On 06.09.18 at 14:08, <andrew.cooper3@citrix.com> wrote:
>>>> @@ -2059,11 +2058,10 @@ csched_dump_pcpu(const struct scheduler *ops, int cpu)
>>>>      spc = CSCHED_PCPU(cpu);
>>>>      runq = &spc->runq;
>>>>  
>>>> -    cpumask_scnprintf(cpustr, sizeof(cpustr), per_cpu(cpu_sibling_mask, cpu));
>>>> -    printk("CPU[%02d] nr_run=%d, sort=%d, sibling=%s, ",
>>>> -           cpu, spc->nr_runnable, spc->runq_sort_last, cpustr);
>>>> -    cpumask_scnprintf(cpustr, sizeof(cpustr), per_cpu(cpu_core_mask, cpu));
>>>> -    printk("core=%s\n", cpustr);
>>>> +    printk("CPU[%02d] nr_run=%d, sort=%d, sibling=%*pb, core=%*pb\n",
>>>> +           cpu, spc->nr_runnable, spc->runq_sort_last,
>>>> +           nr_cpu_ids, per_cpu(cpu_sibling_mask, cpu),
>>>> +           nr_cpu_ids, per_cpu(cpu_core_mask, cpu));
>>> Strictly speaking here and elsewhere you should wrap the CPU mask
>>> accesses in cpumask_bits().
>>
>> Why? Its barely used, and is another example of a helper which only adds
>> to code volume.
> 
> If anyone added (e.g. for debugging) a leading field to struct cpumask,
> your code would break, while all code anywhere else would still be fine.

Do all other current users use cpumask_bits() for dereferencing?

I took Andy's "Its[sic] barely used" comment to mean there were lots of
other places which also just passed a cpumask_t pointer directly into
something expecting a bitmap.  If all other use cases either use
cpumask_bits() or ->bits, then we should do the same here.  If there are
lots of places where we assume (void *)mask == (void *)mask->bits, then
we should probably document that the structure should match that (and
maybe add a BUILD_BUG_ON() if we can manage it).

 -George

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