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

[Tolga Dalman]

Michael,

given the approach is accepted by Carlos and Roland, I have
some minor textual suggestions for the patch itself.

On 06/26/2015 10:05 PM, Michael Kerrisk (man-pages) wrote:
> --- a/man2/sched_setaffinity.2
> +++ b/man2/sched_setaffinity.2
> @@ -223,6 +223,47 @@ system call returns the size (in bytes) of the
>  .I cpumask_t
>  data type that is used internally by the kernel to
>  represent the CPU set bit mask.
> +.SS Handling systems with more than 1024 CPUs

What if the system has exactly 1024 CPUs ?
Suggestion: systems with 1024 or more CPUs

> +The
> +.I cpu_set_t
> +data type used by glibc has a fixed size of 128 bytes,
> +meaning that the maximum CPU number that can be represented is 1023.
> +.\" FIXME . See https://sourceware.org/bugzilla/show_bug.cgi?id=15630
> +.\" and https://sourceware.org/ml/libc-alpha/2013-07/msg00288.html

No objection, although I have never really noticed external references
in man-pages (esp. web refs). Shouldn't these be generally avoided ?
(and yes, I have noticed the FIXME)

> +If the system has more than 1024 CPUs, then calls of the form:

1024 or more CPUs.

> +
> +    sched_getaffinity(pid, sizeof(cpu_set_t), &mask);
> +
> +will fail with the error
> +.BR EINVAL ,
> +the error produced by the underlying system call for the case where the
> +.I mask
> +size specified in
> +.I cpusetsize
> +is smaller than the size of the affinity mask used by the kernel.
> +.PP
> +The underlying system calls (which represent CPU masks as bit masks of type
> +.IR "unsigned long\ *" )
> +impose no restriction on the size of the mask.
> +To handle systems with more than 1024 CPUs, one must dynamically allocate the
> +.I mask
> +argument using
> +.BR CPU_ALLOC (3)

I would rewrite the sentence to avoid "one must".

> +and manipulate the mask using the "_S" macros described in

and manipulate the macros ending with "_S" as described in

> +.BR CPU_ALLOC (3).
> +Using an allocation based on the number of online CPUs:
> +
> +    cpu_set_t *mask = CPU_ALLOC(CPU_ALLOC_SIZE(
> +                                sysconf(_SC_NPROCESSORS_CONF)));
> +
> +is probably sufficient, although the value returned by the
> +.BR sysconf ()
> +call can in theory change during the lifetime of the process.
> +Alternatively, one can obtain a value that is guaranteed to be stable for

Like above, I would replace "one can obtain a value" by "a value can be obtained".

> +the lifetime of the process by proby for the size of the required mask using

s/proby/probing/.

> +.BR sched_getaffinity ()
> +calls with increasing mask sizes until the call does not fail with the error
> +.BR EINVAL .

I would replace "until the call does not fail with error ..." by "while the call succeeds".

Also, the sentence too long, IMHO.

Best regards
Tolga Dalman