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 <t...@linutronix.de> CC: linux-kernel@vger.kernel.org Signed-off-by: Peter Xu <pet...@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