Re: What *is* the API for sched_getaffinity? Should sched_getaffinity always succeed when using cpu_set_t?

[KOSAKI Motohiro <kosaki dot motohiro at gmail dot com>]

Hi

> There are 3 ways to determine the correct size of the possible
> cpu mask size:
>
> (a) Read it from sysfs /sys/devices/system/cpu/online, which
> has the actual number of possibly online cpus.
>
> (b) Interpret /proc/cpuinfo or /proc/stat.
>
> (c) Call the kernel syscall sched_getaffinity with increasingly
> larger values for cpusetsize in an attempt to manually
> determine the cpu mask size.
>
> Methods (a) and (b) are already used by sysconf(_SC_PROCESSORS_ONLN)
> to determine the value to return.

Nit.

sysconf(_SC_NPROCESSORS_ONLN) returns number of online cpus. not possible cpus.

online cpus: number of _now_ onlined cpus
possible cpus: number of maximum cpus on the system. (i.e. depend on arch and
firmware and ACPI table)

/sys/devices/system/cpu/possible returns possible cpu mask. AFAIK, unfortunately,
sysconf() doesn't support to get possible cpus.


> Static applications can't easily be fixed to work around
> this problem. The only solution there is to have the kernel
> stop returning EINVAL and instead do what glibc does which
> is to copy only the part of the buffer that the user requested.
> However, doing that would break existing glibc's which rely
> on EINVAL to compute the mask size. Therefore changing the
> kernel semantics are not a good solution (except on a
> system-by-system basis in the extreme case where a single
> static application was being supported).
>
> Step (s3) ensures that using a fixed cpu_set_t size works
> when you are booted on hardware that has more than 1024
> possible cpus.
>
> Unfortunately it breaks the recommended pattern of using
> sched_getaffinity and looking for EINVAL to determine the
> size of the mask, but this was never a method that glibc
> documented or supported. The patched man page has the
> starting buffer size of 1024, so at least such a pattern
> would allow access to the first 1024 cpus. It is strongly
> recommended that users use sysconf to determine the number
> of possible cpus. This patch to the linux kernel man pages
> has been removed in future versions of RHEL.

If glibc need any kernel change, I'd like to handle it. Please
let me know if needed.