MINOR: config: disable busy polling on old processes

in the context of seamless reload and busy polling, older processes will
create unecessary cpu conflicts; we can assume there is no need for busy
polling for old processes which are waiting to be terminated.

This patch is not a bug fix itself but might be a good stability
improvment when you are un the context of frequent seamless reloads with
a high "hard-stop-after" value; for that reasons I think this patch
should be backported in all 2.x versions.

Signed-off-by: William Dauchy <w.dauchy@criteo.com>

doc/configuration.txt

@@ -1476,6 +1476,10 @@ busy-polling
   prefixing it with the "no" keyword. It is ignored by the "select" and
   "poll" pollers.
 
+  This option is automatically disabled on old processes in the context of
+  seamless reload; it avoids too much cpu conflicts when multiple processes
+  stay around for some time waiting for the end of their current connections.
+
 max-spread-checks <delay in milliseconds>
   By default, haproxy tries to spread the start of health checks across the
   smallest health check interval of all the servers in a farm. The principle is

src/proxy.c

@@ -1116,6 +1116,8 @@ void soft_stop(void)
 	struct task *task;
 
 	stopping = 1;
+	/* disable busy polling to avoid cpu eating for the new process */
+	global.tune.options &= ~GTUNE_BUSY_POLLING;
 	if (tick_isset(global.hard_stop_after)) {
 		task = task_new(MAX_THREADS_MASK);
 		if (task) {