Re: [PATCH 3/4] linux: Use /sys/devices/system/cpu/possible on __get_nprocs_conf

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella via Libc-alpha:

> On 06/05/2021 10:51, Florian Weimer wrote:
>> * Adhemerval Zanella via Libc-alpha:
>> 
>>> On 05/05/2021 15:06, Florian Weimer wrote:
>>>> * Florian Weimer:
>>>>
>>>>> * Adhemerval Zanella via Libc-alpha:
>>>>>
>>>>>> +      /* The entry is in the form of '[cpuX]-[cpuY]'.  */
>>>>>> +      char buf[2 * INT_STRLEN_BOUND (unsigned int) + 1];
>>>>>> +
>>>>>> +      ssize_t n = __read_nocancel (fd, buf, sizeof (buf));
>>>>>> +      if (n > 0)
>>>>>> +	{
>>>>>> +	  buf[n] = '\0';
>>>>>> +
>>>>>> +	  /* Start on the right, to find highest node number.  */
>>>>>> +	  int m = 1;
>>>>>> +	  while (--n)
>>>>>> +	    {
>>>>>> +	      if ((buf[n] == ',') || (buf[n] == '-'))
>>>>>> +		break;
>>>>>> +	      /* Ignore '\n'  */
>>>>>> +	      if (! isdigit (buf[n]))
>>>>>> +		continue;
>>>>>> +	      result += (buf[n] - '0') * m;
>>>>>> +	      m *= 10;
>>>>>> +	    }
>>>>>> +	}
>>>>>> +
>>>>>> +      __close_nocancel (fd);
>>>>>> +      return result + 1;
>>>>>>      }
>>>>>
>>>>> I think the /online and /possible files have the same layout, so you
>>>>> could use both.
>>>>
>>>> Sorry, I meant to write: “so you could use *one parser for* both”
>>>
>>> I am not following, the second patch in this set *removed* the 
>>> /sys/devices/system/cpu/online parsing in favor of the sched_getaffinity.
>>> So now it only parses /sys/devices/system/cpu/possible and fallbacks
>>> to sched_getaffinity.
>> 
>> Oh, right.  So you removed the old implementation and bring bug a
>> slightly different new one.  Got it.
>
> Right, are you ok with the patch then?

The new parser cannot handle gaps or ranges that do not start at 0, I
think.  The old parser could cope with that.  The kernel data structures
support gaps in the possible CPU mask.  I don't know if they occur in
practice, but firmware quirks in this area aren't exactly rare (e.g.,
single-socket systems which report hundreds of hot-pluggable CPU cores).

Thanks,
Florian