glibc: Remove CPU set size checking from affinity functions [BZ #19143]

The current situation, briefly stated, is this: glibc tries to guess the kernel CPU set size and rejects attempts to specify an affinity mask which is larger than that, but it does not work, and glibc and the kernel still silently accept CPU affinity masks with invalid bits, without returning an error. The glibc check does not provide any value to applications, it just adds pointless complexity to the library. Therefore, I want to remove it from glibc.

Quoted 7 lines

The current situation, briefly stated, is this: glibc tries to guess the kernel CPU set size and rejects attempts to specify an affinity mask which is larger than that, but it does not work, and glibc and the kernel still silently accept CPU affinity masks with invalid bits, without returning an error. The glibc check does not provide any value to applications, it just adds pointless complexity to the library. Therefore, I want to remove it from glibc.

checking the validty of the cpuset mask at the time of setting attributes seems fundamentally wrong to me. can't the cpuset change too between the check and the actual use ?

patch makes sense to me

Review

Quoted 1 line

(quoted diff) +1 -0

+	  abort ();

why does the test call abort instead of exit ?

abort writes to stderr.

* Mike Frysinger:

Quoted 1 line

abort writes to stderr.

I don't think so, “Aborted” is printed by the shell.

I still believe this is the right course of action.

This is a straight rebase of an old glibc patch, this time with a bug and a NEWS entry.

The goal of this patch is to remove the CPU set size checking in user space from the glibc system call wrappers for the sched_setaffinity system call (that is, sched_setaffinity and pthread_setaffinity_np).

The previous discussion is here:

<https://sourceware.org/ml/libc-alpha/2015-05/msg00449.html>

I do not have any new arguments to bring forward. I can try to explain what is going on in a few different words, though.

The current situation, briefly stated, is this: glibc tries to guess the kernel CPU set size and rejects attempts to specify an affinity mask which is larger than that, but it does not work, and glibc and the kernel still silently accept CPU affinity masks with invalid bits, without returning an error. The glibc check does not provide any value to applications, it just adds pointless complexity to the library. Therefore, I want to remove it from glibc.

Remove CPU set size checking from affinity functions [BZ #19143]

With current kernel versions, the check does not reliably detect that unavailable CPUs are requested, for these reasons:

(1) The kernel will silently ignore non-allowed CPUs.

(2) Similarly, CPU bits which lack an online CPU (possible CPUs) are ignored.

(3) The existing probing code assumes that the CPU mask size is a power of two and at least 1024. Neither it has to be a power of two, nor is the minimum possible value 1024, so the value determined is often too large, resulting in incorrect false negatives.

The kernel will still return EINVAL if no CPU in the requested set remains which can run the current thread after the affinity change.

Applications which care about the exact affinity mask will have to query it using sched_getaffinity after setting it. Due to the effects above, this commit does not change this.

2015-10-16 Florian Weimer <[email protected]>

* nptl/check-cpuset.h: Remove.
* nptl/pthread_attr_setaffinity.c (__pthread_attr_setaffinity_new):
Remove CPU set size check.
* nptl/pthread_setattr_default_np.c (pthread_setattr_default_np):
Likewise.
* sysdeps/unix/sysv/linux/check-cpuset.h: Remove.
* sysdeps/unix/sysv/linux/pthread_setaffinity.c (__kernel_cpumask_size, __determine_cpumask_size): Remove. (__pthread_setaffinity_new): Remove CPU set size check.
* sysdeps/unix/sysv/linux/sched_setaffinity.c

(__kernel_cpumask_size): Remove. (__sched_setaffinity_new): Remove CPU set size check.
* manual/threads.texi (Default Thread Attributes): Remove stale reference to check_cpuset_attr, determine_cpumask_size in comment.
* posix/Makefile (tests): Add tst-affinity.
* posix/tst-affinity.c: New file.
* nptl/Makefile (tests): Add tst-thread-affinity.
* nptl/tst-thread-affinity.c: New file.

NEWS +0 -0

index 861027a..d012c82 100644
@@ -20,11 +20,19 @@ Version 2.23
   18961, 18966, 18967, 18969, 18970, 18977, 18980, 18981, 18985, 19003,
   19007, 19012, 19016, 19018, 19032, 19046, 19049, 19050, 19059, 19071,

NEWS +1 -0

+
 * The obsolete header <regexp.h> has been removed.  Programs that require
   this header must be updated to use <regex.h> instead.
 

manual/threads.texi +0 -0

index 4d080d4..00cc725 100644

manual/threads.texi +0 -0

 @c  check_stacksize_attr ok
 @c  lll_lock @asulock @aculock
 @c  free dup @ascuheap @acsmem

nptl/Makefile +0 -0

index 311b1a7..e172a0a 100644

nptl/Makefile +0 -0

 xtests = tst-setuid1 tst-setuid1-static tst-setuid2 \
 	tst-mutexpp1 tst-mutexpp6 tst-mutexpp10
 test-srcs = tst-oddstacklimit

nptl/check-cpuset.h deleted +0 -22

deleted file mode 100644
index 315bdf2..0000000
@@ -1,32 +0,0 @@
-/* Validate cpu_set_t values for NPTL.  Stub version.
-   Copyright (C) 2015 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 <errno.h>
-
-/* Returns 0 if CS and SZ are valid values for the cpuset and cpuset size
-   respectively.  Otherwise it returns an error number.  */

nptl/pthread_attr_setaffinity.c +0 -1

index 7a127b8..571835d 100644
@@ -23,7 +23,6 @@
 #include <string.h>
 #include <pthreadP.h>
 #include <shlib-compat.h>
-#include <check-cpuset.h>
 
 
 int

nptl/pthread_attr_setaffinity.c +0 -1

-
       if (iattr->cpusetsize != cpusetsize)
 	{
 	  void *newp = (cpu_set_t *) realloc (iattr->cpuset, cpusetsize);

nptl/pthread_setattr_default_np.c +0 -1

index 457a467..1a661f1 100644
@@ -21,7 +21,6 @@
 #include <pthreadP.h>
 #include <assert.h>
 #include <string.h>
-#include <check-cpuset.h>
 
 
 int

nptl/tst-thread-affinity.c new file +22 -0

+   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 <errno.h>
+#include <pthread.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <sys/time.h>
+

nptl/tst-thread-affinity.c new file +12 -0

+
+#include "../posix/tst-affinity.c"
+
+static int still_running;
+static int failed;
+
+static void *
+thread_burn_one_cpu (void *closure)
+{
+  int cpu = (uintptr_t) closure;
+  while (__atomic_load_n (&still_running, __ATOMIC_RELAXED) == 0)
+    {

nptl/tst-thread-affinity.c new file +21 -0

+  for (int pass = 1; pass <= 2; ++pass)
+    {
+      time_t start = time (NULL);
+      while (time (NULL) - start < 3)
+	{
+	  int cpu = sched_getcpu ();
+	  if (cpu > param->conf->last_cpu
+	      || !CPU_ISSET_S (cpu, CPU_ALLOC_SIZE (param->conf->set_size),
+			       param->initial_set))
+	    {
+	      printf ("FAIL: Unpinned thread %d ran on impossible CPU %d\n",
+		      param->thread, cpu);
+	      __atomic_store_n (&failed, 1, __ATOMIC_RELAXED);
+	      return NULL;
+	    }
+	  CPU_SET_S (cpu, CPU_ALLOC_SIZE (param->conf->set_size),
+		     param->seen_set);
+	  if (pass == 1)
+	    sched_yield ();
+	}
+    }

nptl/tst-thread-affinity.c new file +5 -0

+  for (pthread_t *p = pinned_first; p < pinned_last; ++p)
+    {
+      int cpu = p - pinned_first;
+      if (!CPU_ISSET_S (cpu, CPU_ALLOC_SIZE (conf->set_size), set))
+	continue;

nptl/tst-thread-affinity.c new file +6 -0

+
+  for (struct burn_thread *p = other_first; p < other_last; ++p)
+    {
+      int cpu = p - other_first;
+      if (!CPU_ISSET_S (cpu, CPU_ALLOC_SIZE (conf->set_size), set))
+	continue;

nptl/tst-thread-affinity.c new file +31 -0

+}
+
+/* Tries to check that the initial set of CPUs is complete and that
+   the main thread will not run on any other threads.  */
+static bool
+early_test (struct conf *conf)
+{
+  pthread_t *pinned_threads
+    = calloc (conf->last_cpu + 1, sizeof (*pinned_threads));
+  struct burn_thread *other_threads
+    = calloc (conf->last_cpu + 1, sizeof (*other_threads));
+  cpu_set_t *initial_set = CPU_ALLOC (conf->set_size);
+  cpu_set_t *scratch_set = CPU_ALLOC (conf->set_size);
+
+  if (pinned_threads == NULL || other_threads == NULL
+      || initial_set == NULL || scratch_set == NULL)
+    {
+      puts ("Memory allocation failure");
+      return false;
+    }
+  if (getaffinity (CPU_ALLOC_SIZE (conf->set_size), initial_set) < 0)
+    {
+      printf ("pthread_getaffinity_np failed: %m\n");
+      return false;
+    }
+  for (int cpu = 0; cpu <= conf->last_cpu; ++cpu)
+    {
+      if (!CPU_ISSET_S (cpu, CPU_ALLOC_SIZE (conf->set_size), initial_set))
+	continue;
+      other_threads[cpu].conf = conf;
+      other_threads[cpu].initial_set = initial_set;

nptl/tst-thread-affinity.c new file +29 -0

+
+  pthread_attr_t attr;
+  int ret = pthread_attr_init (&attr);
+  if (ret != 0)
+    {
+      printf ("pthread_attr_init failed: %s\n", strerror (ret));
+      return false;
+    }
+
+  /* Spawn a thread pinned to each available CPU.  */
+  for (int cpu = 0; cpu <= conf->last_cpu; ++cpu)
+    {
+      if (!CPU_ISSET_S (cpu, CPU_ALLOC_SIZE (conf->set_size), initial_set))
+	continue;
+      CPU_ZERO_S (CPU_ALLOC_SIZE (conf->set_size), scratch_set);
+      CPU_SET_S (cpu, CPU_ALLOC_SIZE (conf->set_size), scratch_set);
+      ret = pthread_attr_setaffinity_np
+	(&attr, CPU_ALLOC_SIZE (conf->set_size), scratch_set);
+      if (ret != 0)
+	{
+	  printf ("pthread_attr_setaffinity_np for CPU %d failed: %s\n",
+		  cpu, strerror (ret));
+	  stop_and_join_threads (conf, initial_set,
+				 pinned_threads, pinned_threads + cpu,
+				 NULL, NULL);
+	  return false;
+	}
+      ret = pthread_create (pinned_threads + cpu, &attr,
+			    thread_burn_one_cpu, (void *) (uintptr_t) cpu);

nptl/tst-thread-affinity.c new file +9 -0

+
+  /* Spawn another set of threads running on all CPUs.  */
+  for (int cpu = 0; cpu <= conf->last_cpu; ++cpu)
+    {
+      if (!CPU_ISSET_S (cpu, CPU_ALLOC_SIZE (conf->set_size), initial_set))
+	continue;
+      ret = pthread_create (&other_threads[cpu].self, NULL,
+			    thread_burn_any_cpu, other_threads + cpu);
+      if (ret != 0)

nptl/tst-thread-affinity.c new file +26 -0

+
+  /* Main thread.  */
+  struct burn_thread main_thread;
+  main_thread.conf = conf;
+  main_thread.initial_set = initial_set;
+  main_thread.seen_set = scratch_set;
+  main_thread.thread = -1;
+  CPU_ZERO_S (CPU_ALLOC_SIZE (conf->set_size), main_thread.seen_set);
+  thread_burn_any_cpu (&main_thread);
+  stop_and_join_threads (conf, initial_set,
+			 pinned_threads,
+			 pinned_threads + conf->last_cpu + 1,
+			 other_threads, other_threads + conf->last_cpu + 1);
+
+  printf ("Main thread ran on %d CPU(s) of %d available CPU(s)\n",
+	  CPU_COUNT_S (CPU_ALLOC_SIZE (conf->set_size), scratch_set),
+	  CPU_COUNT_S (CPU_ALLOC_SIZE (conf->set_size), initial_set));
+  CPU_ZERO_S (CPU_ALLOC_SIZE (conf->set_size), scratch_set);
+  for (int cpu = 0; cpu <= conf->last_cpu; ++cpu)
+    {
+      if (!CPU_ISSET_S (cpu, CPU_ALLOC_SIZE (conf->set_size), initial_set))
+	continue;
+      CPU_OR_S (CPU_ALLOC_SIZE (conf->set_size),
+		scratch_set, scratch_set, other_threads[cpu].seen_set);
+      CPU_FREE (other_threads[cpu].seen_set);
+    }

posix/tst-affinity.c new file +26 -0

+/* Copyright (C) 2015 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/>.  */
+
+/* This file is included by nptl/tst-thread-affinity.c to test the
+   pthread variants of the functions.  */
+
+#include <errno.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdbool.h>
+#include <stdio.h>
+

posix/tst-affinity.c new file +13 -0

+  for (int num_cpus = 64; num_cpus <= INT_MAX / 2; num_cpus += 64)
+    {
+      cpu_set_t *set = CPU_ALLOC (num_cpus);
+      size_t size = CPU_ALLOC_SIZE (num_cpus);
+
+      if (set == NULL)
+	{
+	  printf ("CPU_ALLOC(%d) failed\n", num_cpus);
+	  return -1;
+	}
+      if (GETAFFINITY (size, set) == 0)
+	{
+	  CPU_FREE (set);

posix/tst-affinity.c new file +11 -0

+  puts ("Cannot find maximum CPU number");
+  return -1;
+}
+
+static int
+find_last_cpu (const cpu_set_t *set, size_t size)
+{
+  size_t cpus_found = 0;
+  size_t total_cpus = CPU_COUNT_S (size, set);
+  int last_cpu = -1;
+

posix/tst-affinity.c new file +18 -0

+
+static void
+setup_conf (struct conf *conf)
+{
+  *conf = (struct conf) {-1, -1};
+  conf->set_size = find_set_size ();
+  if (conf->set_size > 0)
+    {
+      cpu_set_t *set = CPU_ALLOC (conf->set_size);
+
+      if (set == NULL)
+	{
+	  printf ("CPU_ALLOC (%d) failed\n", conf->set_size);
+	  CPU_FREE (set);
+	  return;
+	}
+      if (GETAFFINITY (CPU_ALLOC_SIZE (conf->set_size), set) < 0)
+	{

posix/tst-affinity.c new file +93 -0

+  if (size < conf->set_size)
+    {
+      printf ("Test not run for CPU set size %zu\n", size);
+      return true;
+    }
+
+  cpu_set_t *initial_set = CPU_ALLOC (size);
+  cpu_set_t *set2 = CPU_ALLOC (size);
+  cpu_set_t *active_cpu_set = CPU_ALLOC (size);
+
+  if (initial_set == NULL || set2 == NULL || active_cpu_set == NULL)
+    {
+      printf ("size %zu: CPU_ALLOC failed\n", size);
+      return false;
+    }
+  size = CPU_ALLOC_SIZE (size);
+
+  if (GETAFFINITY (size, initial_set) < 0)
+    {
+      printf ("size %zu: getaffinity: %m\n", size);
+      return false;
+    }
+  if (SETAFFINITY (size, initial_set) < 0)
+    {
+      printf ("size %zu: setaffinity: %m\n", size);
+      return true;
+    }
+
+  /* Use one-CPU set to test switching between CPUs.  */
+  int last_active_cpu = -1;
+  for (int cpu = 0; cpu <= conf->last_cpu; ++cpu)
+    {
+      int active_cpu = sched_getcpu ();
+      if (last_active_cpu >= 0 && last_active_cpu != active_cpu)
+	{
+	  printf ("Unexpected CPU %d, expected %d\n",
+		  active_cpu, last_active_cpu);
+	  return false;
+	}
+
+      if (!CPU_ISSET_S (cpu, size, initial_set))
+	continue;
+      last_active_cpu = cpu;
+
+      CPU_ZERO_S (size, active_cpu_set);
+      CPU_SET_S (cpu, size, active_cpu_set);
+      if (SETAFFINITY (size, active_cpu_set) < 0)
+	{
+	  printf ("size %zu: setaffinity (%d): %m\n", size, cpu);
+	  return false;
+	}
+      active_cpu = sched_getcpu ();
+      if (active_cpu != cpu)
+	{
+	  printf ("Unexpected CPU %d, expected %d\n", active_cpu, cpu);
+	  return false;
+	}
+      if (GETAFFINITY (size, set2) < 0)
+	{
+	  printf ("size %zu: getaffinity (2): %m\n", size);
+	  return false;
+	}
+      if (!CPU_EQUAL_S (size, active_cpu_set, set2))
+	{
+	  printf ("size %zu: CPU sets do not match\n", size);
+	  return false;
+	}
+    }
+
+  if (SETAFFINITY (size, initial_set) < 0)
+    {
+      printf ("size %zu: setaffinity (3): %m\n", size);
+      return false;
+    }
+  if (GETAFFINITY (size, set2) < 0)
+    {
+      printf ("size %zu: getaffinity (3): %m\n", size);
+      return false;
+    }
+  if (!CPU_EQUAL_S (size, initial_set, set2))
+    {
+      printf ("size %zu: CPU sets do not match (2)\n", size);
+      return false;
+    }
+
+  CPU_FREE (initial_set);
+  CPU_FREE (set2);
+  CPU_FREE (active_cpu_set);
+
+  return true;
+}
+
+static int

posix/tst-affinity.c new file +10 -0

+
+  struct conf conf;
+  setup_conf (&conf);
+  printf ("Detected CPU set size (in bits): %d\n", conf.set_size);
+  printf ("Maximum test CPU: %d\n", conf.last_cpu);
+  if (conf.set_size < 0 || conf.last_cpu < 0)
+    return 1;
+
+  if (!EARLY_TEST (&conf))
+    return 1;

posix/tst-affinity.c new file +4 -0

+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"

sysdeps/unix/sysv/linux/check-cpuset.h deleted +0 -38

deleted file mode 100644
index 1d55e0b..0000000
@@ -1,48 +0,0 @@
-/* Validate cpu_set_t values for NPTL.  Linux version.
-   Copyright (C) 2002-2015 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 <pthread.h>
-#include <errno.h>
-
-
-/* Defined in pthread_setaffinity.c.  */
-extern size_t __kernel_cpumask_size attribute_hidden;
-extern int __determine_cpumask_size (pid_t tid);
-
-/* Returns 0 if CS and SZ are valid values for the cpuset and cpuset size
-   respectively.  Otherwise it returns an error number.  */
-static inline int
-check_cpuset_attr (const cpu_set_t *cs, const size_t sz)
-{
-  if (__kernel_cpumask_size == 0)
-    {
-      int res = __determine_cpumask_size (THREAD_SELF->tid);
-      if (res)
-	return res;
-    }
-

sysdeps/unix/sysv/linux/pthread_setaffinity.c +0 -21

index e891818..2ebf09d 100644
@@ -23,62 +23,14 @@
 #include <shlib-compat.h>
 
 
-size_t __kernel_cpumask_size attribute_hidden;
-
-
-/* Determine the size of cpumask_t in the kernel.  */
-int
-__determine_cpumask_size (pid_t tid)
-{
-  size_t psize;
-  int res;
-
-  for (psize = 128; ; psize *= 2)
-    {
-      char buf[psize];
-      INTERNAL_SYSCALL_DECL (err);
-
-      res = INTERNAL_SYSCALL (sched_getaffinity, err, 3, tid, psize, buf);
-      if (INTERNAL_SYSCALL_ERROR_P (res, err))
-	{
-	  if (INTERNAL_SYSCALL_ERRNO (res, err) != EINVAL)
-	    return INTERNAL_SYSCALL_ERRNO (res, err);
-	}

sysdeps/unix/sysv/linux/pthread_setaffinity.c +0 -7

-
 int
 __pthread_setaffinity_new (pthread_t th, size_t cpusetsize,
 			   const cpu_set_t *cpuset)
 {
   const struct pthread *pd = (const struct pthread *) th;
-
   INTERNAL_SYSCALL_DECL (err);
   int res;
 
-  if (__glibc_unlikely (__kernel_cpumask_size == 0))
-    {
-      res = __determine_cpumask_size (pd->tid);
-      if (res != 0)
-	return res;

sysdeps/unix/sysv/linux/pthread_setaffinity.c +0 -0

   res = INTERNAL_SYSCALL (sched_setaffinity, err, 3, pd->tid, cpusetsize,
 			  cpuset);
 

sysdeps/unix/sysv/linux/sched_setaffinity.c +0 -26

index b528617..dfddce7 100644
@@ -22,50 +22,13 @@
 #include <unistd.h>
 #include <sys/types.h>
 #include <shlib-compat.h>
-#include <alloca.h>
 
 
 #ifdef __NR_sched_setaffinity
-static size_t __kernel_cpumask_size;
-
 
 int
 __sched_setaffinity_new (pid_t pid, size_t cpusetsize, const cpu_set_t *cpuset)
 {
-  if (__glibc_unlikely (__kernel_cpumask_size == 0))
-    {
-      INTERNAL_SYSCALL_DECL (err);
-      int res;
-
-      size_t psize = 128;
-      void *p = alloca (psize);
-
-      while (res = INTERNAL_SYSCALL (sched_getaffinity, err, 3, getpid (),
-				     psize, p),
-	     INTERNAL_SYSCALL_ERROR_P (res, err)
-	     && INTERNAL_SYSCALL_ERRNO (res, err) == EINVAL)
-	p = extend_alloca (p, psize, 2 * psize);
-
-      if (res == 0 || INTERNAL_SYSCALL_ERROR_P (res, err))
-	{
-	  __set_errno (INTERNAL_SYSCALL_ERRNO (res, err));
-	  return -1;
-	}
-
-      __kernel_cpumask_size = res;
-    }
-

Quoted 7 lines

The current situation, briefly stated, is this: glibc tries to guess the kernel CPU set size and rejects attempts to specify an affinity mask which is larger than that, but it does not work, and glibc and the kernel still silently accept CPU affinity masks with invalid bits, without returning an error. The glibc check does not provide any value to applications, it just adds pointless complexity to the library. Therefore, I want to remove it from glibc.

The point of the code you want to remove was to detect the case where the user set CPU bits outside of the maximum possible number of supported CPUs. AFAIK today that value is CONFIG_NR_CPUS and the various variables that derive from that value.

Can you elaborate some more on any of the false negative cases you think might impact user applications? For example what happens if you use the stock cpu_set_t size? It would seem to me that such a use keeps working and there is no change.

Quoted 6 lines

Remove CPU set size checking from affinity functions [BZ #19143]

With current kernel versions, the check does not reliably detect that unavailable CPUs are requested, for these reasons:

(1) The kernel will silently ignore non-allowed CPUs.

You mean to say that if sched_setaffinity is called with a CPU mask bit set to enabled, but that cpu is not allowed for the process, then it will ignore the setting?

This requires you run sched_getaffinity to
verify what CPUs you're actually set to run on?

Is this because the
cpuset mechanism is merged with sched_setaffinity and overrides it?

Quoted 5 lines

(3) The existing probing code assumes that the CPU mask size is a power of two and at least 1024. Neither it has to be a power of two, nor is the minimum possible value 1024, so the value determined is often too large, resulting in incorrect false negatives.

Could you explain those "false negative" cases again?

The goal is to keep nptl/ free from linux-isms, and AFAICT your test is indeed free of any linux-specific features since your code for finding the size of the cpu mask is generic. Is there anything I might have missed that would make your test linux-specific? Keep in mind that we share nptl/ with the nacl port.

Review

Quoted 3 lines

(quoted diff) +3 -0

+* sched_setaffinity, pthread_setaffinity_np no longer attempt to guess the
+  kernel-internal CPU set size.  This means that requests that change the
+  CPU affinity which failed before will now succeed.  Applications that need

Please provide at least one example of a failure which now succeeds.

Review

Quoted 2 lines

(quoted diff) +2 -0

+/* We wave two loops running for two seconds each.  */
+#define TIMEOUT 8

Why two seconds?

Why two threads?

If the value of 2 seconds is arbitrary please state so in a comment such that future reviewers can adjust it as they see fit without having to review the history of the value.

Review

Quoted 2 lines

+++ b/posix/tst-affinity.c

(quoted diff) +0 -0

@@ -0,0 +1,254 @@

Needs a one line test description with BZ#.

Quoted 4 lines

(quoted diff) +4 -0

+static int
+find_set_size (void)
+{
+  /* We need to use multiples of 64 because otherwise, CPU_ALLOC

Review line 6

Quoted 2 lines

(quoted diff) +2 -0

+     over-allocates, and and we do not see all bits returned by the
+     kernel.  */

How does CPU_ALLOC over-allocating result in not seeing bits from the kernel? Is this because you get over-allocation in CPU_ALLOC, but your own external count of num_cpus would be lower and it's num_cpus you return? Can't you rely on the result of CPU_ALLOC_SIZE and return that?

Why do we do this instead of calling sysconf to get the number of CPUs?

Review line 10

Quoted 4 lines

(quoted diff) +4 -0

+
+static bool
+test_size (const struct conf *conf, size_t size)
+{

Should print PASS:/FAIL: prefix to make grepping easier.

Should do what other tests do e.g. err |= and run each test on a distinct line. Makes it easier to add tests and disable tests while debugging.