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Message-ID: <tip-042c79988172ec394d62ba762f09b77e14ecc5b6@git.kernel.org>
Date: Wed, 3 Jul 2019 13:16:26 -0700
From: tip-bot for Peter Xu <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: mingo@...nel.org, lcapitulino@...hat.com, tglx@...utronix.de,
peterx@...hat.com, mtosatti@...hat.com,
linux-kernel@...r.kernel.org, hpa@...or.com
Subject: [tip:timers/core] timer: Document TIMER_PINNED
Commit-ID: 042c79988172ec394d62ba762f09b77e14ecc5b6
Gitweb: https://git.kernel.org/tip/042c79988172ec394d62ba762f09b77e14ecc5b6
Author: Peter Xu <peterx@...hat.com>
AuthorDate: Fri, 28 Jun 2019 18:59:42 +0800
Committer: Thomas Gleixner <tglx@...utronix.de>
CommitDate: Wed, 3 Jul 2019 22:11:58 +0200
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@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Marcelo Tosatti <mtosatti@...hat.com>
Cc: Luiz Capitulino <lcapitulino@...hat.com>
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
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