[PATCH] linux: fix accuracy of get_nprocs and get_nprocs_conf [BZ #28865]

[Adhemerval Zanella]

On 07/02/2022 08:44, Florian Weimer wrote:
> * Adhemerval Zanella via Libc-alpha:
> 
>> On 05/02/2022 18:24, Dmitry V. Levin wrote:
>>> get_nprocs() and get_nprocs_conf() use various methods to obtain an
>>> accurate number of processors.  Re-introduce __get_nprocs_sched() as
>>> a source of information, and fix the order in which these methods are
>>> used to return the most accurate information.  The primary source of
>>> information used in both functions remains unchanged.
>>>
>>> This also changes __get_nprocs_sched() error return value from 2 to 0,
>>> but all its users are already prepared to handle that.
>>>
>>> Old behavior:
>>>   get_nprocs:
>>>     /sys/devices/system/cpu/online -> /proc/stat -> 2
>>>   get_nprocs_conf:
>>>     /sys/devices/system/cpu/ -> /proc/stat -> 2
>>>
>>> New behavior:
>>>   get_nprocs:
>>>     /sys/devices/system/cpu/online -> sched_getaffinity -> /proc/stat -> 2
>>>   get_nprocs_conf:
>>>     /sys/devices/system/cpu/ -> /proc/stat -> sched_getaffinity -> 2
>>>
>>> Fixes: 342298278e ("linux: Revert the use of sched_getaffinity on get_nproc")
>>> Closes: BZ #28865
>>
>> I think we are circling back on this, on BZ#27645 [1] we changed get_nprocs
>> to use sched_getaffinity and then we have to revert it with BZ#28310 [2] because
>> it introduced regression on some monitoring tools [3].
> 
> But I think using sched_getaffinity as a fallback when /sys and /proc
> are not available makes somse.  It's different form what we did
> temporarily (sched_getaffinity first).

My concern is we start to see BZ#27645 again on environments that filter
out sysfs and provide a synthetic sched_getaffinity.