Re: [musl] [PATCH v3] sysconf: add _SC_NPROCESSORS_CONF support

[Samuel Holland <samuel@sholland.org>]

Hi all,

While this topic is being discussed, I'd like to bring up another possible
method for consideration. It turns out that some files in procfs have contents
depending on the number of {possible,online} CPUs; we can parse them to get the
count we need.

The benefits are:
 - procfs is more likely to be mounted than sysfs, and it is already
   documented as being required by musl.
 - Some of the procfs interfaces have been around longer than the
   sysfs interface.
 - The parsing logic is somewhat simpler than for the files in
   /sys/devices/system/cpu, because we are just counting the number
   of occurrences of some string.

The downsides are:
 - It depends on the stability of the procfs file formats.
 - The kernel uses more CPU to generate the contents of these files.

I looked through the procfs code for uses of for_each_possible_cpu and
for_each_online_cpu, and the best candidates I found are:
 - /proc/softirqs for _SC_NPROCESSORS_CONF. It was added in 2009
   (commit d3d64df21d3d, v2.6.31). Its first line contains a column
   for_each_possible_cpu.
 - /proc/stat for _SC_NPROCESSORS_ONLN. It has been around since
   before git. It contains a line for_each_online_cpu.

Here's an example implementation:

diff --git a/src/conf/sysconf.c b/src/conf/sysconf.c
index 3baaed32..8f787b81 100644
--- a/src/conf/sysconf.c
+++ b/src/conf/sysconf.c
@@ -193,10 +193,27 @@ long sysconf(int name)
 		return SEM_VALUE_MAX;
 	case JT_DELAYTIMER_MAX & 255:
 		return DELAYTIMER_MAX;
-	case JT_NPROCESSORS_CONF & 255:
-	case JT_NPROCESSORS_ONLN & 255: ;
+	case JT_NPROCESSORS_CONF & 255: ;
+		FILE *f = fopen("/proc/softirqs", "rbe");
+		int i, cnt = 0;
+		if (f) {
+			while (fscanf(f, " CPU%u", &i) > 0) ++cnt;
+			fclose(f);
+		}
+		if (cnt)
+			return cnt;
+		/* fallthrough */
+	case JT_NPROCESSORS_ONLN & 255:
+		f = fopen("/proc/stat", "rbe");
+		cnt = 0;
+		if (f) {
+			fscanf(f, "cpu %*[^c]");
+			while (fscanf(f, "cpu%u %*[^c]", &i) > 0) ++cnt;
+			fclose(f);
+		}
+		if (cnt)
+			return cnt;
 		unsigned char set[128] = {1};
-		int i, cnt;
 		__syscall(SYS_sched_getaffinity, 0, sizeof set, set);
 		for (i=cnt=0; i<sizeof set; i++)
 			for (; set[i]; set[i]&=set[i]-1, cnt++);

(Apologies if this gets mangled by my MUA,)

Regards,
Samuel