From 28ef2db8866495150e4260773fead8313f1a5625 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Fri, 28 Jun 2019 18:59:42 +0800 Subject: [PATCH] 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 Signed-off-by: Thomas Gleixner Cc: Marcelo Tosatti Cc: Luiz Capitulino Link: https://lkml.kernel.org/r/20190628105942.14131-1-peterx@redhat.com --- include/linux/timer.h | 27 +++++++++++++++++++-------- 1 file changed, 19 insertions(+), 8 deletions(-) diff --git a/include/linux/timer.h b/include/linux/timer.h index 7b066fd38248..282e4f2a532a 100644 --- a/include/linux/timer.h +++ b/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 -- 2.30.2