uclamp: Fix sched_setattr call

The struct sched_param passed to sched_setattr() for uclamp needs to
have a valid policy and priority set.

Currently, sched_data->prio is set to a special value
THREAD_PRIORITY_UNCHANGED when the phase does not declare a specific
priority, and sched_data->policy is set to "same". This is problematic
as the kernel expects a valid policy and priority.  We currently pass
the special values to the kernel, which makes the syscall fail.

Fix that by maintaining a valid policy and priority in
thread_data_t->curr_sched_data->{policy,prio} and using it where
appropriate.

Author: DouglasRaillard

src/rt-app.c

@@ -929,12 +929,17 @@ static void set_thread_membind(thread_data_t *data, numaset_data_t * numa_data)
  * sched_priority is only meaningful for RT tasks. Otherwise, it must be
  * set to 0 for the setattr syscall to succeed.
  */
-static int __sched_priority(thread_data_t *data, sched_data_t *sched_data)
+static int __sched_priority(thread_data_t *data, sched_data_t *sched_data, int curr_prio)
 {
+	int prio = sched_data->prio;
+
+	if (prio == THREAD_PRIORITY_UNCHANGED)
+		prio = curr_prio;
+
 	switch (sched_data->policy) {
 		case rr:
 		case fifo:
-			return sched_data->prio;
+			return prio;
 	}
 
 	 return 0;
@@ -960,7 +965,13 @@ static bool __set_thread_policy_priority(thread_data_t *data,
 	struct sched_param param;
 	int ret;
 
-	param.sched_priority = __sched_priority(data, sched_data);
+	if (sched_data->prio == THREAD_PRIORITY_UNCHANGED) {
+		log_error("Cannot resolve the priority of the thread %s",
+			  data->name);
+		exit(EXIT_FAILURE);
+	}
+
+	param.sched_priority = __sched_priority(data, sched_data, sched_data->prio);
 
 	ret = pthread_setschedparam(pthread_self(),
 				    sched_data->policy,
@@ -1028,12 +1039,20 @@ static void _set_thread_rt(thread_data_t *data, sched_data_t *sched_data)
 }
 
 /* deadline can't rely on the default __set_thread_policy_priority */
-static void _set_thread_deadline(thread_data_t *data, sched_data_t *sched_data)
+static void _set_thread_deadline(thread_data_t *data, sched_data_t *sched_data,
+				 sched_data_t *curr_sched_data)
 {
 	struct sched_attr sa_params = {0};
 	unsigned int flags = 0;
 	pid_t tid;
 	int ret;
+	int curr_prio;
+
+	if (curr_sched_data)
+		curr_prio = curr_sched_data->prio;
+	else
+		/* The value does not matter as it will not be used */
+		curr_prio = THREAD_PRIORITY_UNCHANGED;
 
 	log_debug("[%d] setting scheduler %s exec %lu, deadline %lu"
 		  " period %lu",
@@ -1051,7 +1070,7 @@ static void _set_thread_deadline(thread_data_t *data, sched_data_t *sched_data)
 	sa_params.size = sizeof(struct sched_attr);
 	sa_params.sched_flags = 0;
 	sa_params.sched_policy = SCHED_DEADLINE;
-	sa_params.sched_priority = __sched_priority(data, sched_data);
+	sa_params.sched_priority = __sched_priority(data, sched_data, curr_prio);
 	sa_params.sched_runtime = sched_data->runtime;
 	sa_params.sched_deadline = sched_data->deadline;
 	sa_params.sched_period = sched_data->period;
@@ -1066,19 +1085,39 @@ static void _set_thread_deadline(thread_data_t *data, sched_data_t *sched_data)
 	}
 }
 
-static void _set_thread_uclamp(thread_data_t *data, sched_data_t *sched_data)
+static void _set_thread_uclamp(thread_data_t *data, sched_data_t *sched_data, sched_data_t *curr_sched_data)
 {
 	struct sched_attr sa_params = {0};
 	unsigned int flags = 0;
 	pid_t tid;
 	int ret;
+	policy_t policy;
+	int curr_prio;
 
 	if ((sched_data->util_min == -2 &&
 	     sched_data->util_max == -2))
 		    return;
 
-	sa_params.sched_policy = sched_data->policy;
-	sa_params.sched_priority = __sched_priority(data, sched_data);
+	if (curr_sched_data) {
+		if (sched_data->policy == same)
+			policy = curr_sched_data->policy;
+		else
+			policy = sched_data->policy;
+
+		curr_prio = curr_sched_data->prio;
+	} else {
+		curr_prio = THREAD_PRIORITY_UNCHANGED;
+	}
+
+
+	if (policy == same) {
+		log_error("Cannot resolve the policy of the thread %s",
+			  data->name);
+		exit(EXIT_FAILURE);
+	}
+
+	sa_params.sched_policy = policy;
+	sa_params.sched_priority = __sched_priority(data, sched_data, curr_prio);
 	sa_params.size = sizeof(struct sched_attr);
 	sa_params.sched_flags = SCHED_FLAG_KEEP_ALL;
 	tid = gettid();
@@ -1114,37 +1153,51 @@ static void _set_thread_uclamp(thread_data_t *data, sched_data_t *sched_data)
 
 static void set_thread_param(thread_data_t *data, sched_data_t *sched_data)
 {
+	sched_data_t *curr_sched_data = data->curr_sched_data;
+
 	if (!sched_data)
 		return;
 
-	if (data->curr_sched_data == sched_data)
-		return;
+	if (curr_sched_data) {
+		if (curr_sched_data == sched_data)
+			return;
+
+		if (sched_data->prio == curr_sched_data->prio)
+			sched_data->prio = THREAD_PRIORITY_UNCHANGED;
 
-	/* if no policy is specified, reuse the previous one */
-	if ((sched_data->policy == same) && data->curr_sched_data)
-		sched_data->policy = data->curr_sched_data->policy;
+		/* if no policy is specified, reuse the previous one */
+		if (sched_data->policy == same)
+			sched_data->policy = curr_sched_data->policy;
+        }
 
 	switch (sched_data->policy) {
 		case rr:
 		case fifo:
 			_set_thread_rt(data, sched_data);
-			_set_thread_uclamp(data, sched_data);
+			_set_thread_uclamp(data, sched_data, curr_sched_data);
 			break;
 		case other:
 		case idle:
 			_set_thread_cfs(data, sched_data);
-			_set_thread_uclamp(data, sched_data);
+			_set_thread_uclamp(data, sched_data, curr_sched_data);
 			data->lock_pages = 0; /* forced off */
 			break;
 		case deadline:
-			_set_thread_deadline(data, sched_data);
+			_set_thread_deadline(data, sched_data, curr_sched_data);
 			break;
 		default:
 			log_error("Unknown scheduling policy %d",
 				  sched_data->policy);
 			exit(EXIT_FAILURE);
 	}
 
+	/* Ensure we have an actual valid priority in curr_sched_data at all
+	 * time, since it could be needed in a later phase for sched_setattr()
+	 * syscall
+	 */
+	if (sched_data->prio == THREAD_PRIORITY_UNCHANGED)
+		sched_data->prio = curr_sched_data->prio;
+
 	data->curr_sched_data = sched_data;
 }