[PATCH 0/3] Revert the use of sched_getaffinity on get_nproc

[PATCH 0/3] Revert the use of sched_getaffinity on get_nproc

[Adhemerval Zanella]

The use of sched_getaffinity on get_nproc and
sysconf (_SC_NPROCESSORS_ONLN) done in 903bc7dcc2acafc40 (BZ #27645)
breaks the top command in common hypervisor configurations and also
other monitoring tools.

This patchset revert to previous strategy to obtain the system-wide
information first from sysfs, and then on procfs as a fallback.

The sched_getaffinity is still used on malloc code since it makes more
sense to get the per-process value to tune the maximum arena value.

Adhemerval Zanella (3):
  misc: Add __get_nprocs_sched
  linux: Simplify get_nprocs
  linux: Revert the use of sched_getaffinity on get_nproc (BZ #28310)

 include/sys/sysinfo.h                 |   7 +-
 malloc/arena.c                        |   2 +-
 misc/getsysstats.c                    |   6 +
 posix/Makefile                        |   3 +-
 sysdeps/mach/getsysstats.c            |   6 +
 sysdeps/unix/sysv/linux/getsysstats.c | 192 ++++++++++++++++++++------
 6 files changed, 167 insertions(+), 49 deletions(-)

-- 
2.30.2

[PATCH 1/3] misc: Add __get_nprocs_sched

[Adhemerval Zanella]

This is an internal function meant to return the number of avaliable
processor where the process can scheduled, different than the
__get_nprocs which returns a the system available online CPU.

The Linux implementation currently only calls __get_nprocs(), which
in tuns calls sched_getaffinity.
---
 include/sys/sysinfo.h                 | 7 ++++++-
 malloc/arena.c                        | 2 +-
 misc/getsysstats.c                    | 6 ++++++
 sysdeps/mach/getsysstats.c            | 6 ++++++
 sysdeps/unix/sysv/linux/getsysstats.c | 6 ++++++
 5 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/include/sys/sysinfo.h b/include/sys/sysinfo.h
index 7388356a19..c490561581 100644
--- a/include/sys/sysinfo.h
+++ b/include/sys/sysinfo.h
@@ -9,10 +9,15 @@
 extern int __get_nprocs_conf (void);
 libc_hidden_proto (__get_nprocs_conf)
 
-/* Return number of available processors.  */
+/* Return number of available processors (not all of them will be
+   available to the caller process).  */
 extern int __get_nprocs (void);
 libc_hidden_proto (__get_nprocs)
 
+/* Return the number of available processors which the process can
+   be scheduled.  */
+extern int __get_nprocs_sched (void) attribute_hidden;
+
 /* Return number of physical pages of memory in the system.  */
 extern long int __get_phys_pages (void);
 libc_hidden_proto (__get_phys_pages)
diff --git a/malloc/arena.c b/malloc/arena.c
index 4c398753ae..78ef4cf18c 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -878,7 +878,7 @@ arena_get2 (size_t size, mstate avoid_arena)
             narenas_limit = mp_.arena_max;
           else if (narenas > mp_.arena_test)
             {
-              int n = __get_nprocs ();
+              int n = __get_nprocs_sched ();
 
               if (n >= 1)
                 narenas_limit = NARENAS_FROM_NCORES (n);
diff --git a/misc/getsysstats.c b/misc/getsysstats.c
index 2986d62247..5cbba0f9bd 100644
--- a/misc/getsysstats.c
+++ b/misc/getsysstats.c
@@ -44,6 +44,12 @@ weak_alias (__get_nprocs, get_nprocs)
 link_warning (get_nprocs, "warning: get_nprocs will always return 1")
 
 
+int
+__get_nprocs_sched (void)
+{
+  return 1;
+}
+
 long int
 __get_phys_pages (void)
 {
diff --git a/sysdeps/mach/getsysstats.c b/sysdeps/mach/getsysstats.c
index 1267f39da2..cc8023f979 100644
--- a/sysdeps/mach/getsysstats.c
+++ b/sysdeps/mach/getsysstats.c
@@ -62,6 +62,12 @@ __get_nprocs (void)
 libc_hidden_def (__get_nprocs)
 weak_alias (__get_nprocs, get_nprocs)
 
+int
+__get_nprocs_sched (void)
+{
+  return __get_nprocs ();
+}
+
 /* Return the number of physical pages on the system. */
 long int
 __get_phys_pages (void)
diff --git a/sysdeps/unix/sysv/linux/getsysstats.c b/sysdeps/unix/sysv/linux/getsysstats.c
index 8a5d342f0c..e9c0dc4d83 100644
--- a/sysdeps/unix/sysv/linux/getsysstats.c
+++ b/sysdeps/unix/sysv/linux/getsysstats.c
@@ -87,6 +87,12 @@ __get_nprocs (void)
 libc_hidden_def (__get_nprocs)
 weak_alias (__get_nprocs, get_nprocs)
 
+int
+__get_nprocs_sched (void)
+{
+  return __get_nprocs ();
+}
+
 
 /* On some architectures it is possible to distinguish between configured
    and active cpus.  */
-- 
2.30.2

[PATCH 2/3] 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).

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(-)

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/sysdeps/unix/sysv/linux/getsysstats.c b/sysdeps/unix/sysv/linux/getsysstats.c
index e9c0dc4d83..b9710f8319 100644
--- a/sysdeps/unix/sysv/linux/getsysstats.c
+++ b/sysdeps/unix/sysv/linux/getsysstats.c
@@ -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)
-- 
2.30.2

Re: [PATCH 2/3] linux: Simplify get_nprocs

[Florian Weimer]

* 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

Re: [PATCH 2/3] linux: Simplify get_nprocs

[Adhemerval Zanella]

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.

Re: [PATCH 2/3] linux: Simplify get_nprocs

[Florian Weimer]

* 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

[PATCH 3/3] linux: Revert the use of sched_getaffinity on get_nproc (BZ #28310)

[Adhemerval Zanella]

The use of sched_getaffinity on get_nproc and
sysconf (_SC_NPROCESSORS_ONLN) done in 903bc7dcc2acafc40 (BZ #27645)
breaks the top command in common hypervisor configurations and also
other monitoring tools.

The main issue using sched_getaffinity changed the symbols semantic
from system-wide scope of online CPUs to per-process one (which can
be changed with kernel cpusets or book parameters in VM).

This patch reverts mostly of the 903bc7dcc2acafc40, with the
exceptions:

  * No more cached values and atomic updates, since they are inherent
    racy.

  * No /proc/cpuinfo fallback, since /proc/stat is already used and
    it would require to revert more arch-specific code.

  * The alloca is replace with a static buffer of 1024 bytes.

So the implementation first consult the sysfs, and fallbacks to procfs.

Checked on x86_64-linux-gnu.
---
 sysdeps/unix/sysv/linux/getsysstats.c | 140 +++++++++++++++++++++++++-
 1 file changed, 135 insertions(+), 5 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/getsysstats.c b/sysdeps/unix/sysv/linux/getsysstats.c
index b9710f8319..a1f84c01e2 100644
--- a/sysdeps/unix/sysv/linux/getsysstats.c
+++ b/sysdeps/unix/sysv/linux/getsysstats.c
@@ -17,6 +17,8 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <array_length.h>
+#include <assert.h>
+#include <ctype.h>
 #include <dirent.h>
 #include <errno.h>
 #include <ldsodefs.h>
@@ -29,7 +31,7 @@
 #include <sysdep.h>
 
 int
-__get_nprocs (void)
+__get_nprocs_sched (void)
 {
   /* This cannot use malloc because it is used on malloc initialization.  */
   enum { max_num_cpus = 32768 };
@@ -48,14 +50,142 @@ __get_nprocs (void)
      atomics are needed). */
   return 2;
 }
-libc_hidden_def (__get_nprocs)
-weak_alias (__get_nprocs, get_nprocs)
+
+static char *
+next_line (int fd, char *const buffer, char **cp, char **re,
+           char *const buffer_end)
+{
+  char *res = *cp;
+  char *nl = memchr (*cp, '\n', *re - *cp);
+  if (nl == NULL)
+    {
+      if (*cp != buffer)
+        {
+          if (*re == buffer_end)
+            {
+              memmove (buffer, *cp, *re - *cp);
+              *re = buffer + (*re - *cp);
+              *cp = buffer;
+
+              ssize_t n = __read_nocancel (fd, *re, buffer_end - *re);
+              if (n < 0)
+                return NULL;
+
+              *re += n;
+
+              nl = memchr (*cp, '\n', *re - *cp);
+              while (nl == NULL && *re == buffer_end)
+                {
+                  /* Truncate too long lines.  */
+                  *re = buffer + 3 * (buffer_end - buffer) / 4;
+                  n = __read_nocancel (fd, *re, buffer_end - *re);
+                  if (n < 0)
+                    return NULL;
+
+                  nl = memchr (*re, '\n', n);
+                  **re = '\n';
+                  *re += n;
+                }
+            }
+          else
+            nl = memchr (*cp, '\n', *re - *cp);
+
+          res = *cp;
+        }
+
+      if (nl == NULL)
+        nl = *re - 1;
+    }
+
+  *cp = nl + 1;
+  assert (*cp <= *re);
+
+  return res == *re ? NULL : res;
+}
+
 
 int
-__get_nprocs_sched (void)
+__get_nprocs (void)
 {
-  return __get_nprocs ();
+  enum { buffer_size = 1024 };
+  char buffer[buffer_size];
+  char *buffer_end = buffer + buffer_size;
+  char *cp = buffer_end;
+  char *re = buffer_end;
+
+  const int flags = O_RDONLY | O_CLOEXEC;
+  /* This file contains comma-separated ranges.  */
+  int fd = __open_nocancel ("/sys/devices/system/cpu/online", flags);
+  char *l;
+  int result = 0;
+  if (fd != -1)
+    {
+      l = next_line (fd, buffer, &cp, &re, buffer_end);
+      if (l != NULL)
+	do
+	  {
+	    char *endp;
+	    unsigned long int n = strtoul (l, &endp, 10);
+	    if (l == endp)
+	      {
+		result = 0;
+		break;
+	      }
+
+	    unsigned long int m = n;
+	    if (*endp == '-')
+	      {
+		l = endp + 1;
+		m = strtoul (l, &endp, 10);
+		if (l == endp)
+		  {
+		    result = 0;
+		    break;
+		  }
+	      }
+
+	    result += m - n + 1;
+
+	    l = endp;
+	    if (l < re && *l == ',')
+	      ++l;
+	  }
+	while (l < re && *l != '\n');
+
+      __close_nocancel_nostatus (fd);
+
+      if (result > 0)
+	return result;
+    }
+
+  cp = buffer_end;
+  re = buffer_end;
+
+  /* Default to an SMP system in case we cannot obtain an accurate
+     number.  */
+  result = 2;
+
+  /* The /proc/stat format is more uniform, use it by default.  */
+  fd = __open_nocancel ("/proc/stat", flags);
+  if (fd != -1)
+    {
+      result = 0;
+
+      while ((l = next_line (fd, buffer, &cp, &re, buffer_end)) != NULL)
+	/* The current format of /proc/stat has all the cpu* entries
+	   at the front.  We assume here that stays this way.  */
+	if (strncmp (l, "cpu", 3) != 0)
+	  break;
+	else if (isdigit (l[3]))
+	  ++result;
+
+      __close_nocancel_nostatus (fd);
+    }
+
+  return result;
 }
+libc_hidden_def (__get_nprocs)
+weak_alias (__get_nprocs, get_nprocs)
 
 
 /* On some architectures it is possible to distinguish between configured
-- 
2.30.2

Re: [PATCH 3/3] linux: Revert the use of sched_getaffinity on get_nproc (BZ #28310)

[Florian Weimer]

* Adhemerval Zanella:

> +  /* The /proc/stat format is more uniform, use it by default.  */

This comment seems outdated.  Rest looks okay (but I didn't run the full
test suite).

Thanks,
Florian

Re: [PATCH 3/3] linux: Revert the use of sched_getaffinity on get_nproc (BZ #28310)

[Adhemerval Zanella]

On 07/09/2021 06:45, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> +  /* The /proc/stat format is more uniform, use it by default.  */
> 
> This comment seems outdated.  Rest looks okay (but I didn't run the full
> test suite).

Ack, I will remove it.