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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
This commit is contained in:
Peter Xu 2019-06-28 18:59:42 +08:00 committed by Thomas Gleixner
parent dd2cb34861
commit 28ef2db886
1 changed files with 19 additions and 8 deletions
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27
include/linux/timer.h
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@ -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