[2/3] linux: Simplify get_nprocs
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Commit Message
This patch simplifies the memory allocation code and uses the sched
routines instead of reimplement it. This still uses a stack
allocation buffer, so it can be used on malloc initialization code.
Linux currently supports at maximum of 4096 cpus for most architectures:
$ find -iname Kconfig | xargs git grep -A10 -w NR_CPUS | grep -w range
arch/alpha/Kconfig- range 2 32
arch/arc/Kconfig- range 2 4096
arch/arm/Kconfig- range 2 16 if DEBUG_KMAP_LOCAL
arch/arm/Kconfig- range 2 32 if !DEBUG_KMAP_LOCAL
arch/arm64/Kconfig- range 2 4096
arch/csky/Kconfig- range 2 32
arch/hexagon/Kconfig- range 2 6 if SMP
arch/ia64/Kconfig- range 2 4096
arch/mips/Kconfig- range 2 256
arch/openrisc/Kconfig- range 2 32
arch/parisc/Kconfig- range 2 32
arch/riscv/Kconfig- range 2 32
arch/s390/Kconfig- range 2 512
arch/sh/Kconfig- range 2 32
arch/sparc/Kconfig- range 2 32 if SPARC32
arch/sparc/Kconfig- range 2 4096 if SPARC64
arch/um/Kconfig- range 1 1
arch/x86/Kconfig-# [NR_CPUS_RANGE_BEGIN ... NR_CPUS_RANGE_END] range.
arch/x86/Kconfig- range NR_CPUS_RANGE_BEGIN NR_CPUS_RANGE_END
arch/xtensa/Kconfig- range 2 32
With x86 supporting 8192:
arch/x86/Kconfig
976 config NR_CPUS_RANGE_END
977 int
978 depends on X86_64
979 default 8192 if SMP && CPUMASK_OFFSTACK
980 default 512 if SMP && !CPUMASK_OFFSTACK
981 default 1 if !SMP
So using a maximum of 32k cpu should cover all cases (and I would
expect once we start to have many more CPUs that Linux would provide
a more straightforward way to query for such information).
A test is added to check if sched_getaffinity can successfully return
with large buffers.
Checked on x86_64-linux-gnu and i686-linux-gnu.
---
posix/Makefile | 3 +-
sysdeps/unix/sysv/linux/getsysstats.c | 64 ++++++---------------------
2 files changed, 16 insertions(+), 51 deletions(-)
Comments
* Adhemerval Zanella:
> + /* This cannot use malloc because it is used on malloc initialization. */
> + enum { max_num_cpus = 32768 };
> + size_t cpu_bits_size = CPU_ALLOC_SIZE (max_num_cpus);
> + __cpu_mask cpu_bits[cpu_bits_size / sizeof (__cpu_mask)];
I think cpu_bits_size should be an enum constant as well, to avoid
creating a VLA.
Thanks,
Florian
* Adhemerval Zanella:
> diff --git a/posix/Makefile b/posix/Makefile
> index 059efb3cd2..09460a28e8 100644
> --- a/posix/Makefile
> +++ b/posix/Makefile
> @@ -107,7 +107,8 @@ tests := test-errno tstgetopt testfnm runtests runptests \
> tst-sysconf-empty-chroot tst-glob_symlinks tst-fexecve \
> tst-glob-tilde test-ssize-max tst-spawn4 bug-regex37 \
> bug-regex38 tst-regcomp-truncated tst-spawn-chdir \
> - tst-wordexp-nocmd tst-execveat tst-spawn5
> + tst-wordexp-nocmd tst-execveat tst-spawn5 \
> + tst-sched_getaffinity
>
tst-sched_getaffinity is missing from this patch.
Thanks,
Florian
On 07/09/2021 06:30, Florian Weimer wrote:
> * Adhemerval Zanella:
>
>> + /* This cannot use malloc because it is used on malloc initialization. */
>> + enum { max_num_cpus = 32768 };
>> + size_t cpu_bits_size = CPU_ALLOC_SIZE (max_num_cpus);
>> + __cpu_mask cpu_bits[cpu_bits_size / sizeof (__cpu_mask)];
>
> I think cpu_bits_size should be an enum constant as well, to avoid
> creating a VLA.
Indeed, I will fix it.
@@ -107,7 +107,8 @@ tests := test-errno tstgetopt testfnm runtests runptests \
tst-sysconf-empty-chroot tst-glob_symlinks tst-fexecve \
tst-glob-tilde test-ssize-max tst-spawn4 bug-regex37 \
bug-regex38 tst-regcomp-truncated tst-spawn-chdir \
- tst-wordexp-nocmd tst-execveat tst-spawn5
+ tst-wordexp-nocmd tst-execveat tst-spawn5 \
+ tst-sched_getaffinity
# Test for the glob symbol version that was replaced in glibc 2.27.
ifeq ($(have-GLIBC_2.26)$(build-shared),yesyes)
@@ -28,61 +28,25 @@
#include <sys/sysinfo.h>
#include <sysdep.h>
-/* Compute the population count of the entire array. */
-static int
-__get_nprocs_count (const unsigned long int *array, size_t length)
-{
- int count = 0;
- for (size_t i = 0; i < length; ++i)
- if (__builtin_add_overflow (count, __builtin_popcountl (array[i]),
- &count))
- return INT_MAX;
- return count;
-}
-
-/* __get_nprocs with a large buffer. */
-static int
-__get_nprocs_large (void)
-{
- /* This code cannot use scratch_buffer because it is used during
- malloc initialization. */
- size_t pagesize = GLRO (dl_pagesize);
- unsigned long int *page = __mmap (0, pagesize, PROT_READ | PROT_WRITE,
- MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
- if (page == MAP_FAILED)
- return 2;
- int r = INTERNAL_SYSCALL_CALL (sched_getaffinity, 0, pagesize, page);
- int count;
- if (r > 0)
- count = __get_nprocs_count (page, pagesize / sizeof (unsigned long int));
- else if (r == -EINVAL)
- /* One page is still not enough to store the bits. A more-or-less
- arbitrary value. This assumes t hat such large systems never
- happen in practice. */
- count = GLRO (dl_pagesize) * CHAR_BIT;
- else
- count = 2;
- __munmap (page, GLRO (dl_pagesize));
- return count;
-}
-
int
__get_nprocs (void)
{
- /* Fast path for most systems. The kernel expects a buffer size
- that is a multiple of 8. */
- unsigned long int small_buffer[1024 / CHAR_BIT / sizeof (unsigned long int)];
- int r = INTERNAL_SYSCALL_CALL (sched_getaffinity, 0,
- sizeof (small_buffer), small_buffer);
+ /* This cannot use malloc because it is used on malloc initialization. */
+ enum { max_num_cpus = 32768 };
+ size_t cpu_bits_size = CPU_ALLOC_SIZE (max_num_cpus);
+ __cpu_mask cpu_bits[cpu_bits_size / sizeof (__cpu_mask)];
+ int r = INTERNAL_SYSCALL_CALL (sched_getaffinity, 0, cpu_bits_size,
+ cpu_bits);
if (r > 0)
- return __get_nprocs_count (small_buffer, r / sizeof (unsigned long int));
+ return CPU_COUNT_S (cpu_bits_size, (cpu_set_t*) cpu_bits);
else if (r == -EINVAL)
- /* The kernel requests a larger buffer to store the data. */
- return __get_nprocs_large ();
- else
- /* Some other error. 2 is conservative (not a uniprocessor
- system, so atomics are needed). */
- return 2;
+ /* The input buffer is still not enough to store the number of cpus. This
+ is an arbitrary values assuming such systems should be rare and there
+ is no offline cpus. */
+ return max_num_cpus;
+ /* Some other error. 2 is conservative (not a uniprocessor system, so
+ atomics are needed). */
+ return 2;
}
libc_hidden_def (__get_nprocs)
weak_alias (__get_nprocs, get_nprocs)