The flag is very easy to understand but the interface is not,
especially the assumption that add_timer_on() must be used to
guarantee a correct behavior.
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 <t...@linutronix.de>
CC: linux-kernel@vger.kernel.org
Signed-off-by: Peter Xu <pet...@redhat.com>
---
include/linux/timer.h | 23 +++++++++++++++--------
1 file changed, 15 insertions(+), 8 deletions(-)
diff --git a/include/linux/timer.h b/include/linux/timer.h
index 7b066fd38248..4a3408a05f86 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -36,19 +36,26 @@ 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 timer
+ * migration so it will always be run on a static cpu that was
+ * specified. Note: neither timer_setup() nor mod_timer() will
+ * guarantee correct pinning of timers. One should always use
+ * add_timer_on() when arm the timer to guarantee that the timer will
+ * be pinned to the target CPU correctly.
*/
#define TIMER_CPUMASK 0x0003FFFF
#define TIMER_MIGRATING 0x00040000
--
2.17.1
