[3/3] linux: Revert the use of sched_getaffinity on get_nproc (BZ #28310)
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Commit Message
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(-)
Comments
* 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This comment seems outdated.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
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 This comment seems outdated.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.
@@ -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