sched/kconfig: add INITIAL_PID_LIMIT

This adds a config for initial pid table length so that to reduce
reallocations when num of threads are known by design.

Signed-off-by: Yanfeng Liu <[email protected]>

Author: yf13

sched/Kconfig

@@ -1937,3 +1937,14 @@ config GROUP_KILL_CHILDREN_TIMEOUT_MS
 		> 0 means wait timeout
 		= 0 means don't do kill signal
 
+config INITIAL_PID_LIMIT
+	int "Initial length of pid hash table"
+	default 4 if DEFAULT_SMALL
+	default 16 if !DEFAULT_SMALL
+	---help---
+		This specifies the initial length of pid table, it shall be
+		power of two. Note this doesn't prevent the system from
+		expanding it if more pids are needed. If the max number
+		of threads in your system is known by design, setting this
+		to it can avoid unnecessary reallocations of the table after
+		boot.

sched/init/nx_start.c

@@ -45,6 +45,7 @@
 #include <nuttx/binfmt/binfmt.h>
 #include <nuttx/drivers/drivers.h>
 #include <nuttx/init.h>
+#include <nuttx/lib/math32.h>
 
 #include "task/task.h"
 #include "sched/sched.h"
@@ -623,7 +624,9 @@ void nx_start(void)
 
   /* Initialize the logic that determine unique process IDs. */
 
-  nxsched_npidhash() = 4;
+  static_assert(IS_POWER_OF_2(CONFIG_INITIAL_PID_LIMIT));
+
+  nxsched_npidhash() = CONFIG_INITIAL_PID_LIMIT;
   while (nxsched_npidhash() <= CONFIG_SMP_NCPUS)
     {
       nxsched_npidhash() <<= 1;