logging: wake up log process thread only when log message happen

Periodically wake up log process thread consume more power if system
already in sleep or deep sleep state. With the help of added logging
timer and semaphore, log process thread is woken up only when there
is logging message.

Fixes: #19982.

Signed-off-by: Wentong Wu <wentong.wu@intel.com>

subsys/logging/log_core.c

@@ -48,6 +48,8 @@ struct log_strdup_buf {
 #define LOG_STRDUP_POOL_BUFFER_SIZE \
 	(sizeof(struct log_strdup_buf) * CONFIG_LOG_STRDUP_BUF_COUNT)
 
+K_SEM_DEFINE(log_process_thread_sem, 0, 1);
+
 static const char *log_strdup_fail_msg = "<log_strdup alloc failed>";
 struct k_mem_slab log_strdup_pool;
 static u8_t __noinit __aligned(sizeof(u32_t))
@@ -61,6 +63,8 @@ static atomic_t buffered_cnt;
 static atomic_t dropped_cnt;
 static k_tid_t proc_tid;
 
+static struct k_timer log_process_thread_timer;
+
 static u32_t dummy_timestamp(void);
 static timestamp_get_t timestamp_func = dummy_timestamp;
 
@@ -89,10 +93,14 @@ static inline void msg_finalize(struct log_msg *msg,
 		key = irq_lock();
 		(void)log_process(false);
 		irq_unlock(key);
+	} else if (proc_tid != NULL && buffered_cnt == 1) {
+		k_timer_start(&log_process_thread_timer,
+			CONFIG_LOG_PROCESS_THREAD_SLEEP_MS, 0);
 	} else if (CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD) {
 		if ((buffered_cnt == CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD) &&
 		    (proc_tid != NULL)) {
-			k_wakeup(proc_tid);
+			k_timer_stop(&log_process_thread_timer);
+			k_sem_give(&log_process_thread_sem);
 		}
 	}
 }
@@ -379,7 +387,7 @@ static void thread_set(k_tid_t process_tid)
 	if (CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD &&
 	    process_tid &&
 	    buffered_cnt >= CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD) {
-		k_wakeup(proc_tid);
+		k_sem_give(&log_process_thread_sem);
 	}
 }
 
@@ -648,6 +656,14 @@ void log_backend_enable(struct log_backend const *const backend,
 	log_backend_id_set(backend, id);
 	backend_filter_set(backend, level);
 	log_backend_activate(backend, ctx);
+
+	/* Wakeup logger thread after attaching first backend. It might be
+	 * blocked with log messages pending.
+	 */
+	if (!backend_attached) {
+		k_sem_give(&log_process_thread_sem);
+	}
+
 	backend_attached = true;
 }
 
@@ -953,6 +969,11 @@ void log_hexdump_from_user(struct log_msg_ids src_level, const char *metadata,
 }
 #endif /* defined(CONFIG_NO_OPTIMIZATIONS) */
 
+static void log_process_thread_timer_expiry_fn(struct k_timer *timer)
+{
+	k_sem_give(&log_process_thread_sem);
+}
+
 static void log_process_thread_func(void *dummy1, void *dummy2, void *dummy3)
 {
 	__ASSERT_NO_MSG(log_backend_count_get() > 0);
@@ -962,7 +983,7 @@ static void log_process_thread_func(void *dummy1, void *dummy2, void *dummy3)
 
 	while (true) {
 		if (log_process(false) == false) {
-			k_sleep(CONFIG_LOG_PROCESS_THREAD_SLEEP_MS);
+			k_sem_take(&log_process_thread_sem, K_FOREVER);
 		}
 	}
 }
@@ -975,6 +996,8 @@ static int enable_logger(struct device *arg)
 	ARG_UNUSED(arg);
 
 	if (IS_ENABLED(CONFIG_LOG_PROCESS_THREAD)) {
+		k_timer_init(&log_process_thread_timer,
+				log_process_thread_timer_expiry_fn, NULL);
 		/* start logging thread */
 		k_thread_create(&logging_thread, logging_stack,
 				K_THREAD_STACK_SIZEOF(logging_stack),