MINOR: cpu-topo: implement a sorting mechanism for CPU index

CPU selection will be performed by sorting CPUs according to
various criteria. For dumps however, that's really not convenient
and we'll need to reorder the CPUs according to their index only.
This is what the new function cpu_reorder_by_index() does. It's
called  in thread_detect_count() before dumping the CPU topology.

include/haproxy/cpu_topo.h

@@ -43,4 +43,14 @@ int cpu_map_configured(void);
  */
 void cpu_dump_topology(const struct ha_cpu_topo *topo);
 
+/* re-order a CPU topology array by CPU index only, to undo the function above,
+ * in case other calls need to be made on top of this.
+ */
+void cpu_reorder_by_index(struct ha_cpu_topo *topo, int entries);
+
+/* Functions used by qsort to compare hardware CPUs (not meant to be used from
+ * outside cpu_topo).
+ */
+int _cmp_cpu_index(const void *a, const void *b);
+
 #endif /* _HAPROXY_CPU_TOPO_H */

src/cpu_topo.c

@@ -221,6 +221,32 @@ void cpu_dump_topology(const struct ha_cpu_topo *topo)
 	}
 }
 
+/* function used by qsort to re-arrange CPUs by index only, to restore original
+ * ordering.
+ */
+int _cmp_cpu_index(const void *a, const void *b)
+{
+	const struct ha_cpu_topo *l = (const struct ha_cpu_topo *)a;
+	const struct ha_cpu_topo *r = (const struct ha_cpu_topo *)b;
+
+	/* next, IDX, so that SMT ordering is preserved */
+	if (l->idx >= 0 && l->idx < r->idx)
+		return -1;
+	if (l->idx > r->idx && r->idx >= 0)
+		return  1;
+
+	/* exactly the same (e.g. absent, should not happend) */
+	return 0;
+}
+
+/* re-order a CPU topology array by CPU index only. This is mostly used before
+ * listing CPUs regardless of their characteristics.
+ */
+void cpu_reorder_by_index(struct ha_cpu_topo *topo, int entries)
+{
+	qsort(topo, entries, sizeof(*topo), _cmp_cpu_index);
+}
+
 /* returns an optimal maxcpus for the current system. It will take into
  * account what is reported by the OS, if any, otherwise will fall back
  * to the cpuset size, which serves as an upper limit in any case.

src/thread.c

@@ -1695,8 +1695,10 @@ void thread_detect_count(void)
 	}
 
 #if defined(USE_THREAD) && defined(USE_CPU_AFFINITY)
-	if (global.tune.debug & GDBG_CPU_AFFINITY)
+	if (global.tune.debug & GDBG_CPU_AFFINITY) {
+		cpu_reorder_by_index(ha_cpu_topo, cpu_topo_maxcpus);
 		cpu_dump_topology(ha_cpu_topo);
+	}
 #endif
 	return;
 }