Sigh.... I forgot much of what I learned as I wrote the CPU_SET(3) page many years ago. Revised patch below. On 06/26/2015 04:28 PM, Michael Kerrisk (man-pages) wrote: > Carlos, > > On 07/23/2013 12:34 AM, Carlos O'Donell wrote: >> On 07/22/2013 05:43 PM, Roland McGrath wrote: >>>> I can fix the glibc manual. A 'configured' CPU is one that the OS >>>> can bring online. >>> >>> Where do you get this definition, in the absence of a standard that >>> specifies _SC_NPROCESSORS_CONF? The only definition I've ever known for >>> _SC_NPROCESSORS_CONF is a value that's constant for at least the life of >>> the process (and probably until reboot) that is the upper bound for what >>> _SC_NPROCESSORS_ONLN might ever report. If the implementation for Linux is >>> inconsistent with that definition, then it's just a bug in the implementation. >> >> Let me reiterate my understanding such that you can help me clarify >> exactly my interpretation of the glibc manual wording regarding the >> two existing constants. >> >> The reality of the situation is that the linux kernel as an abstraction >> presents the following: >> >> (a) The number of online cpus. >> - Changes dynamically. >> - Not constant for the life of the process, but pretty constant. >> >> (b) The number of configured cpus. >> - The number of detected cpus that the OS could access. >> - Some of them may be offline for various reasons. >> - Changes dynamically with hotplug. >> >> (c) The number of possible CPUs the OS or hardware can support. >> - The internal software infrastructure is designed to support at >> most this many cpus. >> - Constant for the uptime of the system. >> - May be tied in some way to the hardware. >> >> On Linux, glibc currently maps _SC_NPROCESSORS_CONF to (b) via >> /sys/devices/system/cpu/cpu*, and _SC_NPROCESSORS_ONLN to (a) via >> /sys/devices/system/cpu/online. >> >> The problem is that sched_getaffinity and sched_setaffinity only cares >> about (c) since the size of the kernel affinity mask is of size (c). >> >> What Motohiro-san was requesting was that the manual should make it clear >> that _SC_NPROCESSORS_CONF is distinct from (c) which is an OS limit that >> the user doesn't know. >> >> We need not expose (c) as a new _SC_* constant since it's not really >> required, since glibc's sched_getaffinity and sched_setaffinity could >> hide the fact that (c) exists from userspace (and that's what I suggest >> should happen). >> >> Does that clarify my statement? > > It's a long time since the last activity in this discussion, and I see that > https://sourceware.org/bugzilla/show_bug.cgi?id=15630 > remains open, I propose to apply the patch below to the > sched_setattr/sched_getattr man page. Seem okay? > > Cheers, > > Michael > > > --- a/man2/sched_setaffinity.2 > +++ b/man2/sched_setaffinity.2 > @@ -333,6 +334,57 @@ main(int argc, char *argv[]) > } > } > .fi > +.SH BUGS > +The glibc > +.BR sched_setaffinity () > +and > +.BR sched_getaffinity () > +wrapper functions do not handle systems with more than 1024 CPUs. > +.\" FIXME . See https://sourceware.org/bugzilla/show_bug.cgi?id=15630 > +.\" and https://sourceware.org/ml/libc-alpha/2013-07/msg00288.html > +The > +.I cpu_set_t > +data type used by glibc has a fixed size of 128 bytes, > +meaning that the the maximum CPU number that can be represented is 1023. > +If the system has more than 1024 CPUs, then: > +.IP * 3 > +The > +.BR sched_setaffinity () > +.I mask > +argument is not capable of representing the excess CPUs. > +.IP * > +Calls of the form: > + > + sched_getaffinity(pid, sizeof(cpu_set_t), &mask); > + > +will fail with 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 workaround for this problem is to fall back to the use of the > +underlying system call (via > +.BR syscall (2)), > +passing > +.I mask > +arguments of a sufficient size. > +Using a value based on the number of online CPUs: > + > + (sysconf(_SC_NPROCESSORS_CONF) / (sizeof(unsigned long) * 8) + 1) > + * sizeof(unsigned long) > + > +is probably sufficient as the size of the mask, > +although the value returned by the > +.BR sysconf () > +call can in theory change during the lifetime of the process. > +Alternatively, one can probe for the size of the required mask using raw > +.BR sched_getaffinity () > +system calls with increasing mask sizes > +until the call does not fail with the error > +.BR EINVAL . > .SH SEE ALSO > .ad l > .nh Okay -- scratch the above. How about the patch below. Cheers, Michael --- 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 +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. +.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) +and manipulate the mask using the "_S" macros 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 +the lifetime of the process by proby for the size of the required mask using +.BR sched_getaffinity () +calls with increasing mask sizes until the call does not fail with the error +.BR EINVAL . .SH EXAMPLE The program below creates a child process. The parent and child then each assign themselves to a specified CPU -- 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
