[PATCH] linux: Simplify get_nprocs

[Adhemerval Zanella]

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).

Checked on x86_64-linux-gnu and i686-linux-gnu.
---
 sysdeps/unix/sysv/linux/getsysstats.c | 64 ++++++---------------------
 1 file changed, 14 insertions(+), 50 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/getsysstats.c b/sysdeps/unix/sysv/linux/getsysstats.c
index 1391e360b8..e752b0a111 100644
--- a/sysdeps/unix/sysv/linux/getsysstats.c
+++ b/sysdeps/unix/sysv/linux/getsysstats.c
@@ -29,61 +29,25 @@
 #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)
-- 
2.30.2