From 1c59ff16d3c24ddc5ac8f5735f352aefecb763be Mon Sep 17 00:00:00 2001
From: Andrew Clayton <a.clayton@nginx.com>
Date: Fri, 26 Sep 2025 21:59:04 +0100
Subject: [PATCH] Better available cpu count determination.

With 'worker_processes auto;' the number of worker processes to create
is generally determined at runtime via sysconf(_SC_NPROCESSORS_ONLN);

However this value does not take into account any restrictions that have
been put in place via mechanisms like cpuset(7)'s and/or
sched_setaffinity(2).

So for example while a system may have 18 "cpus" available as returned
via sysconf(_SC_NPROCESSORS_ONLN) the actual number of allowed to run on
cpus may be much lower.

E.g.

On a 18 core system, with 'worker_processes auto;' nginx will create 18
worker processes (+ the master). E.g.

  $ pgrep -c nginx
  19

If however nginx has been restricted to run on just four. We will still
create 18 worker processes. E.g.

  $ taskset -a -c 0-3 ./sbin/nginx -c conf/minimal.conf
  $ pgrep -c nginx
  19

This can happen for example with 'docker --cpuset-cpus=', where you may
end up with many more worker processes than runnable cpus.

This commit uses sched_getaffinity() (where available, at least Linux &
FreeBSD) to determine the number of actual available runnable cpus.

So in the above example we now get

  $ taskset -a -c 0-3 ./sbin/nginx -c conf/minimal.conf
  $ pgrep -c nginx
  5

Four worker processes plus the master.

Closes: https://github.com/nginx/nginx/issues/855
---
 src/os/unix/ngx_posix_init.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/src/os/unix/ngx_posix_init.c b/src/os/unix/ngx_posix_init.c
index 6ac931c0c..0d1ecf928 100644
--- a/src/os/unix/ngx_posix_init.c
+++ b/src/os/unix/ngx_posix_init.c
@@ -64,6 +64,28 @@ ngx_os_init(ngx_log_t *log)
     }
 #endif
 
+#if (NGX_HAVE_SCHED_GETAFFINITY)
+    if (ngx_ncpu > 0) {
+        int         err;
+        size_t      sz;
+        cpu_set_t  *mask;
+
+        mask = CPU_ALLOC(ngx_ncpu);
+        if (mask == NULL) {
+            return NGX_ERROR;
+        }
+
+        sz = CPU_ALLOC_SIZE(ngx_ncpu);
+
+        err = sched_getaffinity(0, sz, mask);
+        if (err == 0) {
+            ngx_ncpu = CPU_COUNT_S(sz, mask);
+        }
+
+        CPU_FREE(mask);
+    }
+#endif
+
     if (ngx_ncpu < 1) {
         ngx_ncpu = 1;
     }