On Wed, Apr 03, 2019 at 11:08:09PM +0300, Alexey Dobriyan wrote: > Currently there is no easy way to get the number of CPUs on the system. And this patch doesn't change that :-) Still, it does the right thing and I like it. The point is that nr_cpu_ids is the length of the bitmap, but does not contain information on how many CPUs are in the system. Consider the case where the bitmap is sparse. > Applications are divided into 2 groups: > One group allocates buffer and call sched_getaffinity(2) once. It works > but either underallocate or overallocates and in the future such application > will become buggy as Linux will start working on even more SMP-ier systems. > > Glibc in particular shipped with 1024 CPUs support maximum at some point > which is quite surprising as glibc maitainers should know better. > > Another group dynamically grow buffer until cpumask fits. This is > inefficient as multiple system calls are done. > > Nobody seems to parse "/sys/devices/system/cpu/possible". > Even if someone does, parsing sysfs is much slower than necessary. True; but I suppose glibc already does lots of that anyway, right? It does contain the right information. > Patch overloads sched_getaffinity(len=0) to simply return "nr_cpu_ids". > This will make gettting CPU mask require at most 2 system calls > and will eliminate unnecessary code. > > len=0 is chosen so that > * passing zeroes is the simplest thing > > syscall(__NR_sched_getaffinity, 0, 0, NULL) > > will simply do the right thing, > > * old kernels returned -EINVAL unconditionally. > > Note: glibc segfaults upon exiting from system call because it tries to > clear the rest of the buffer if return value is positive, so > applications will have to use syscall(3). > Good news is that it proves noone uses sched_getaffinity(pid, 0, NULL). This also needs a manpage update. And I'm missing the libc people on Cc. > Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx> > --- > > kernel/sched/core.c | 3 +++ > 1 file changed, 3 insertions(+) > > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -4942,6 +4942,9 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len, > int ret; > cpumask_var_t mask; > > + if (len == 0) > + return nr_cpu_ids; > + > if ((len * BITS_PER_BYTE) < nr_cpu_ids) > return -EINVAL; > if (len & (sizeof(unsigned long)-1))
