sched: Refine the comment for SCHED_INSTRUMENTATION_EXTERNAL

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

sched/Kconfig

@@ -920,7 +920,16 @@ config SCHED_INSTRUMENTATION_EXTERNAL
 	bool "System performance monitor endpoints are external"
 	default n
 	---help---
-		Monitor only CPUs in the bitset.  Bit 0=CPU0, Bit1=CPU1, etc.
+		When this option is enabled, the board specific logic must implment all
+		callbacks listed in SCHED_INSTRUMENTATION, SCHED_INSTRUMENTATION_CSECTION,
+		SCHED_INSTRUMENTATION_SPINLOCKS, SCHED_INSTRUMENTATION_SYSCALL and
+		SCHED_INSTRUMENTATION_IRQHANDLER. Otherwise the common code will implement
+		these callbacks and packet the arguments into note_ struct. Then the board
+		-specific logic just need to implement one callback:
+
+		  void sched_note_add(FAR const void *note, size_t notelen);
+
+		and send the data to the suitable transport hardware.
 
 config SCHED_INSTRUMENTATION_CPUSET
 	hex "CPU bit set"

sched/sched/Make.defs

@@ -90,8 +90,10 @@ CSRCS += sched_thistask.c
 endif
 
 ifeq ($(CONFIG_SCHED_INSTRUMENTATION),y)
+ifeq ($(CONFIG_SCHED_INSTRUMENTATION_EXTERNAL),)
 CSRCS += sched_note.c
 endif
+endif
 
 ifeq ($(CONFIG_SCHED_CRITMONITOR),y)
 CSRCS += sched_critmonitor.c

sched/sched/sched_note.c

@@ -36,7 +36,6 @@
 
 #include "sched/sched.h"
 
-#if !defined(CONFIG_SCHED_INSTRUMENTATION_EXTERNAL)
 /****************************************************************************
  * Private Types
  ****************************************************************************/
@@ -611,4 +610,3 @@ void sched_note_irqhandler(int irq, FAR void *handler, bool enter)
   note_add((FAR const uint8_t *)&note, sizeof(struct note_irqhandler_s));
 }
 #endif
-#endif /* CONFIG_SCHED_INSTRUMENTATION_EXTERNAL */