Hello Florian, On 22 July 2015 at 18:02, Florian Weimer <fweimer@xxxxxxxxxx> wrote: > On 07/21/2015 05:03 PM, Michael Kerrisk (man-pages) wrote: >> Hello Florian, >> >> Thanks for your comments, and sorry for the delayed follow-up. >> >> On 07/01/2015 02:37 PM, Florian Weimer wrote: >>> On 06/26/2015 10:05 PM, Michael Kerrisk (man-pages) wrote: >>> >>>> +.SS Handling systems with more than 1024 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 >>>> +If the system has more than 1024 CPUs, then calls of the form: >>>> + >>>> + 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. >>> >>> I think it is best to leave this as unspecified as possible. Kernel >>> behavior already changed once, and I can imagine it changing again. >> >> Hmmm. Something needs to be said about what the kernel is doing though. >> Otherwise, it's hard to make sense of this subsection. Did you have a >> suggested rewording that removes the piece you find problematic? > > What about this? > > “If the kernel affinity mask is larger than 1024 then > … > is smaller than the size of the affinity mask used by the kernel. > Depending on the system CPU topology, the kernel affinity mask can > be substantially larger than the number of active CPUs in the system. > ” Looks good. I've taken that. > I.e., make clear that the size of the mask can be quite different from > the CPU count. > >> Handling systems with more than 1024 CPUs >> The underlying system calls (which represent CPU masks as bit >> masks of type unsigned long *) impose no restriction on the >> size of the CPU mask. However, the 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. If the system has >> more than 1024 CPUs, then calls of the form: >> >> sched_getaffinity(pid, sizeof(cpu_set_t), &mask); >> >> will fail with the error EINVAL, the error produced by the >> underlying system call for the case where the mask size speci‐ >> fied in cpusetsize is smaller than the size of the affinity >> mask used by the kernel. >> >> When working on systems with more than 1024 CPUs, one must >> dynamically allocate the mask argument. Currently, the only >> way to do this is by probing for the size of the required mask >> using sched_getaffinity() calls with increasing mask sizes >> (until the call does not fail with the error EINVAL). >> >> Better? > > “more than 1024 CPUs” should be “large [kernel CPU] affinity masks” > throughout. Done. Thanks for your further input. So now we have: C library/kernel differences This manual page describes the glibc interface for the CPU affin‐ ity calls. The actual system call interface is slightly differ‐ ent, with the mask being typed as unsigned long *, reflecting the fact that the underlying implementation of CPU sets is a simple bit mask. On success, the raw sched_getaffinity() system call returns the size (in bytes) of the cpumask_t data type that is used internally by the kernel to represent the CPU set bit mask. Handling systems with large CPU affinity masks The underlying system calls (which represent CPU masks as bit masks of type unsigned long *) impose no restriction on the size of the CPU mask. However, the cpu_set_t data type used by glibc has a fixed size of 128 bytes, meaning that the maximum CPU num‐ ber that can be represented is 1023. If the kernel CPU affinity mask is larger than 1024, then calls of the form: sched_getaffinity(pid, sizeof(cpu_set_t), &mask); will fail with the error EINVAL, the error produced by the under‐ lying system call for the case where the mask size specified in cpusetsize is smaller than the size of the affinity mask used by the kernel. (Depending on the system CPU topology, the kernel affinity mask can be substantially larger than the number of active CPUs in the system.) When working on systems with large kernel CPU affinity masks, one must dynamically allocate the mask argument. Currently, the only way to do this is by probing for the size of the required mask using sched_getaffinity() calls with increasing mask sizes (until the call does not fail with the error EINVAL). Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
