diff --git a/include/logging/log_core.h b/include/logging/log_core.h
index 23839981de6..6fa56254aad 100644
--- a/include/logging/log_core.h
+++ b/include/logging/log_core.h
@@ -630,6 +630,18 @@ void log_dropped(void);
 void __printf_like(2, 3) log_from_user(struct log_msg_ids src_level,
 				       const char *fmt, ...);
 
+/**
+ * @brief Create mask with occurences of a string format specifiers (%s).
+ *
+ * Result is stored as the mask (argument n is n'th bit). Bit is set if string
+ * format specifier was found.
+ *
+ * @param str String.
+ * @param nargs Number of arguments in the string.
+ *
+ * @return Mask with %s format specifiers found.
+ */
+u32_t z_log_get_s_mask(const char *str, u32_t nargs);
 
 /* Internal function used by log_from_user(). */
 __syscall void z_log_string_from_user(u32_t src_level_val, const char *str);
diff --git a/subsys/logging/log_core.c b/subsys/logging/log_core.c
index b384714bb28..6949aa23d05 100644
--- a/subsys/logging/log_core.c
+++ b/subsys/logging/log_core.c
@@ -81,23 +81,7 @@ static u32_t dummy_timestamp(void)
 	return 0;
 }
 
-/**
- * @brief Count number of string format specifiers (%s).
- *
- * Result is stored as the mask (argument n is n'th bit). Bit is set if %s was
- * found.
- *
- * @note Algorithm does not take into account complex format specifiers as they
- *	 hardly used in log messages and including them would significantly
- *	 extended this function which is called on every log message is feature
- *	 is enabled.
- *
- * @param str String.
- * @param nargs Number of arguments in the string.
- *
- * @return Mask with %s format specifiers found.
- */
-static u32_t count_s(const char *str, u32_t nargs)
+u32_t z_log_get_s_mask(const char *str, u32_t nargs)
 {
 	char curr;
 	bool arm = false;
@@ -173,7 +157,7 @@ static void detect_missed_strdup(struct log_msg *msg)
 	}
 
 	msg_str = log_msg_str_get(msg);
-	mask = count_s(msg_str, log_msg_nargs_get(msg));
+	mask = z_log_get_s_mask(msg_str, log_msg_nargs_get(msg));
 
 	while (mask) {
 		idx = 31 - __builtin_clz(mask);
diff --git a/subsys/logging/log_msg.c b/subsys/logging/log_msg.c
index 59b8a21f979..929d6c569e8 100644
--- a/subsys/logging/log_msg.c
+++ b/subsys/logging/log_msg.c
@@ -103,17 +103,38 @@ static void cont_free(struct log_msg_cont *cont)
 
 static void msg_free(struct log_msg *msg)
 {
-	u32_t nargs = msg->hdr.params.std.nargs;
+	u32_t nargs = log_msg_nargs_get(msg);
 
 	/* Free any transient string found in arguments. */
 	if (log_msg_is_std(msg) && nargs) {
 		int i;
+		u32_t smask = 0;
 
 		for (i = 0; i < nargs; i++) {
 			void *buf = (void *)log_msg_arg_get(msg, i);
 
 			if (log_is_strdup(buf)) {
-				log_free(buf);
+				if (smask == 0) {
+					/* Do string arguments scan only when
+					 * string duplication candidate detected
+					 * since it is time consuming and free
+					 * can be called from any context when
+					 * log message is being dropped.
+					 */
+					smask = z_log_get_s_mask(
+							log_msg_str_get(msg),
+							nargs);
+					if (smask == 0) {
+						/* if no string argument is
+						 * detected then stop searching
+						 * for candidates.
+						 */
+						break;
+					}
+				}
+				if (smask & BIT(i)) {
+					log_free(buf);
+				}
 			}
 		}
 	} else if (IS_ENABLED(CONFIG_USERSPACE) &&
diff --git a/tests/subsys/logging/log_core/src/log_core_test.c b/tests/subsys/logging/log_core/src/log_core_test.c
index 3b832ef0af2..261468de26a 100644
--- a/tests/subsys/logging/log_core/src/log_core_test.c
+++ b/tests/subsys/logging/log_core/src/log_core_test.c
@@ -477,6 +477,22 @@ static void test_log_msg_dropped_notification(void)
 	k_sched_unlock();
 }
 
+static void test_single_z_log_get_s_mask(const char *str, u32_t nargs,
+					 u32_t exp_mask)
+{
+	u32_t mask = z_log_get_s_mask(str, nargs);
+
+	zassert_equal(mask, exp_mask, "Unexpected mask %x (expected %x)",
+								mask, exp_mask);
+}
+
+static void test_z_log_get_s_mask(void)
+{
+	test_single_z_log_get_s_mask("%d%%%-10s%p%x", 4, 0x2);
+	test_single_z_log_get_s_mask("%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d"
+				     "%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%s",
+				     32, 0x80000000);
+}
 /*
  * Test checks if panic is correctly executed. On panic logger should flush all
  * messages and process logs in place (not in deferred way).
@@ -522,6 +538,7 @@ void test_main(void)
 			 ztest_unit_test(test_log_strdup_detect_miss),
 			 ztest_unit_test(test_strdup_trimming),
 			 ztest_unit_test(test_log_msg_dropped_notification),
+			 ztest_unit_test(test_z_log_get_s_mask),
 			 ztest_unit_test(test_log_panic));
 	ztest_run_test_suite(test_log_list);
 }