[PATCH v2 2/3] linux: Simplify get_nprocs

[Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>]

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 +-
 posix/tst-sched_getaffinity.c         | 47 ++++++++++++++++++
 sysdeps/unix/sysv/linux/getsysstats.c | 68 +++++++--------------------
 3 files changed, 67 insertions(+), 51 deletions(-)
 create mode 100644 posix/tst-sched_getaffinity.c

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
 
 # Test for the glob symbol version that was replaced in glibc 2.27.
 ifeq ($(have-GLIBC_2.26)$(build-shared),yesyes)
diff --git a/posix/tst-sched_getaffinity.c b/posix/tst-sched_getaffinity.c
new file mode 100644
index 0000000000..6d1fdcb05f
--- /dev/null
+++ b/posix/tst-sched_getaffinity.c
@@ -0,0 +1,47 @@
+/* Tests for sched_getaffinity with large buffers.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <array_length.h>
+#include <sched.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+  /* The values are larger than the default cpu_set_t.  */
+  const int bufsize[] = { 1<<11, 1<<12, 1<<13, 1<<14, 1<<15, 1<<16, 1<<17 };
+  int cpucount[array_length(bufsize)];
+
+  for (int i = 0; i < array_length (bufsize); i++)
+    {
+      cpu_set_t *cpuset = CPU_ALLOC (bufsize[i]);
+      TEST_VERIFY (cpuset != NULL);
+      size_t size = CPU_ALLOC_SIZE (bufsize[i]);
+      TEST_COMPARE (sched_getaffinity (0, size, cpuset), 0);
+      cpucount[i] = CPU_COUNT_S (size, cpuset);
+      CPU_FREE (cpuset);
+    }
+
+  for (int i = 0; i < array_length (cpucount) - 1; i++)
+    for (int j = 1; j < array_length (cpucount); j++)
+      TEST_COMPARE (cpucount[i], cpucount[j]);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/unix/sysv/linux/getsysstats.c b/sysdeps/unix/sysv/linux/getsysstats.c
index e9c0dc4d83..1e3d88676d 100644
--- a/sysdeps/unix/sysv/linux/getsysstats.c
+++ b/sysdeps/unix/sysv/linux/getsysstats.c
@@ -28,61 +28,29 @@
 #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);
+  enum
+    {
+      max_num_cpus = 32768,
+      cpu_bits_size = CPU_ALLOC_SIZE (32768)
+    };
+
+  /* This cannot use malloc because it is used on malloc initialization.  */
+  __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