[PATCH v2] timer: document TIMER_PINNED

[Peter Xu <peterx@redhat.com>]

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 current implementation.

For example, currently if we setup a pinned timer but later on we call
mod_timer() upon the pinned timer, the mod_timer() will still try to
run the timer on the current processor and migrate the timer if
necessary.  In other words, the suggested way to arm a pinned timer
should be add_timer_on() always.  mod_timer() can be used in this case
only if current processor is the one that we want to pin the timer on.

Document it a bit with the definition of TIMER_PINNED so that all
future users will use it correctly.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: linux-kernel@vger.kernel.org
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/timer.h | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 7b066fd38248..1b96cc623a12 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -36,19 +36,31 @@ 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 be run on the CPU
+ * when the timer was enqueued.
+ *
+ * Note: Because enqueuing of timers can actually 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.  It is also suggested that the user should always use
+ * add_timer_on() explicitly for pinned timers.
  */
 #define TIMER_CPUMASK		0x0003FFFF
 #define TIMER_MIGRATING		0x00040000
-- 
2.21.0