timer: Document TIMER_PINNED

The flag hints the user that the pinned timers will always be run on a
static CPU (because that should be what "pinned" means...) but that's
not the truth, at least with the current implementation.

For example, currently if a pinned timer is set up but later mod_timer()
upon the pinned timer is invoked, mod_timer() will still try to queue the
timer on the current processor and migrate the timer if necessary.

Document it a bit with the definition of TIMER_PINNED so that all future
users will use it correctly.
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Link: https://lkml.kernel.org/r/20190628105942.14131-1-peterx@redhat.com

include/linux/timer.h

@@ -36,19 +36,30 @@ struct timer_list {
 #define __TIMER_LOCKDEP_MAP_INITIALIZER(_kn)
 #endif
 
-/*
- * A deferrable timer will work normally when the system is busy, but
- * will not cause a CPU to come out of idle just to service it; instead,
- * the timer will be serviced when the CPU eventually wakes up with a
- * subsequent non-deferrable timer.
+/**
+ * @TIMER_DEFERRABLE: A deferrable timer will work normally when the
+ * system is busy, but will not cause a CPU to come out of idle just
+ * to service it; instead, the timer will be serviced when the CPU
+ * eventually wakes up with a subsequent non-deferrable timer.
  *
- * An irqsafe timer is executed with IRQ disabled and it's safe to wait for
- * the completion of the running instance from IRQ handlers, for example,
- * by calling del_timer_sync().
+ * @TIMER_IRQSAFE: An irqsafe timer is executed with IRQ disabled and
+ * it's safe to wait for the completion of the running instance from
+ * IRQ handlers, for example, by calling del_timer_sync().
  *
  * Note: The irq disabled callback execution is a special case for
  * workqueue locking issues. It's not meant for executing random crap
  * with interrupts disabled. Abuse is monitored!
+ *
+ * @TIMER_PINNED: A pinned timer will not be affected by any timer
+ * placement heuristics (like, NOHZ) and will always expire on the CPU
+ * on which the timer was enqueued.
+ *
+ * Note: Because enqueuing of timers can migrate the timer from one
+ * CPU to another, pinned timers are not guaranteed to stay on the
+ * initialy selected CPU.  They move to the CPU on which the enqueue
+ * function is invoked via mod_timer() or add_timer().  If the timer
+ * should be placed on a particular CPU, then add_timer_on() has to be
+ * used.
  */
 #define TIMER_CPUMASK		0x0003FFFF
 #define TIMER_MIGRATING		0x00040000