[PATCH] timers fixes/improvements
This patch tries to solve following problems:
1. del_timer_sync() is racy. The timer can be fired again after
del_timer_sync have checked all cpus and before it will recheck
timer_pending().
2. It has scalability problems. All cpus are scanned to determine
if the timer is running on that cpu.
With this patch del_timer_sync is O(1) and no slower than plain
del_timer(pending_timer), unless it has to actually wait for
completion of the currently running timer.
The only restriction is that the recurring timer should not use
add_timer_on().
3. The timers are not serialized wrt to itself.
If CPU_0 does mod_timer(jiffies+1) while the timer is currently
running on CPU 1, it is quite possible that local interrupt on
CPU_0 will start that timer before it finished on CPU_1.
4. The timers locking is suboptimal. __mod_timer() takes 3 locks
at once and still requires wmb() in del_timer/run_timers.
The new implementation takes 2 locks sequentially and does not
need memory barriers.
Currently ->base != NULL means that the timer is pending. In that case
->base.lock is used to lock the timer. __mod_timer also takes timer->lock
because ->base can be == NULL.
This patch uses timer->entry.next != NULL as indication that the timer is
pending. So it does __list_del(), entry->next = NULL instead of list_del()
when the timer is deleted.
The ->base field is used for hashed locking only, it is initialized
in init_timer() which sets ->base = per_cpu(tvec_bases). When the
tvec_bases.lock is locked, it means that all timers which are tied
to this base via timer->base are locked, and the base itself is locked
too.
So __run_timers/migrate_timers can safely modify all timers which could
be found on ->tvX lists (pending timers).
When the timer's base is locked, and the timer removed from ->entry list
(which means that _run_timers/migrate_timers can't see this timer), it is
possible to set timer->base = NULL and drop the lock: the timer remains
locked.
This patch adds lock_timer_base() helper, which waits for ->base != NULL,
locks the ->base, and checks it is still the same.
__mod_timer() schedules the timer on the local CPU and changes it's base.
However, it does not lock both old and new bases at once. It locks the
timer via lock_timer_base(), deletes the timer, sets ->base = NULL, and
unlocks old base. Then __mod_timer() locks new_base, sets ->base = new_base,
and adds this timer. This simplifies the code, because AB-BA deadlock is not
possible. __mod_timer() also ensures that the timer's base is not changed
while the timer's handler is running on the old base.
__run_timers(), del_timer() do not change ->base anymore, they only clear
pending flag.
So del_timer_sync() can test timer->base->running_timer == timer to detect
whether it is running or not.
We don't need timer_list->lock anymore, this patch kills it.
We also don't need barriers. del_timer() and __run_timers() used smp_wmb()
before clearing timer's pending flag. It was needed because __mod_timer()
did not lock old_base if the timer is not pending, so __mod_timer()->list_add()
could race with del_timer()->list_del(). With this patch these functions are
serialized through base->lock.
One problem. TIMER_INITIALIZER can't use per_cpu(tvec_bases). So this patch
adds global
struct timer_base_s {
spinlock_t lock;
struct timer_list *running_timer;
} __init_timer_base;
which is used by TIMER_INITIALIZER. The corresponding fields in tvec_t_base_s
struct are replaced by struct timer_base_s t_base.
It is indeed ugly. But this can't have scalability problems. The global
__init_timer_base.lock is used only when __mod_timer() is called for the first
time AND the timer was compile time initialized. After that the timer migrates
to the local CPU.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Renaud Lienhart <renaud.lienhart@free.fr>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/include/linux/timer.h b/include/linux/timer.h
index 90db1cc..2e78fed 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -6,45 +6,33 @@
#include <linux/spinlock.h>
#include <linux/stddef.h>
-struct tvec_t_base_s;
+struct timer_base_s;
struct timer_list {
struct list_head entry;
unsigned long expires;
- spinlock_t lock;
unsigned long magic;
void (*function)(unsigned long);
unsigned long data;
- struct tvec_t_base_s *base;
+ struct timer_base_s *base;
};
#define TIMER_MAGIC 0x4b87ad6e
+extern struct timer_base_s __init_timer_base;
+
#define TIMER_INITIALIZER(_function, _expires, _data) { \
.function = (_function), \
.expires = (_expires), \
.data = (_data), \
- .base = NULL, \
+ .base = &__init_timer_base, \
.magic = TIMER_MAGIC, \
- .lock = SPIN_LOCK_UNLOCKED, \
}
-/***
- * init_timer - initialize a timer.
- * @timer: the timer to be initialized
- *
- * init_timer() must be done to a timer prior calling *any* of the
- * other timer functions.
- */
-static inline void init_timer(struct timer_list * timer)
-{
- timer->base = NULL;
- timer->magic = TIMER_MAGIC;
- spin_lock_init(&timer->lock);
-}
+void fastcall init_timer(struct timer_list * timer);
/***
* timer_pending - is a timer pending?
@@ -58,7 +46,7 @@
*/
static inline int timer_pending(const struct timer_list * timer)
{
- return timer->base != NULL;
+ return timer->entry.next != NULL;
}
extern void add_timer_on(struct timer_list *timer, int cpu);
@@ -89,12 +77,12 @@
#ifdef CONFIG_SMP
extern int del_timer_sync(struct timer_list *timer);
- extern int del_singleshot_timer_sync(struct timer_list *timer);
#else
# define del_timer_sync(t) del_timer(t)
-# define del_singleshot_timer_sync(t) del_timer(t)
#endif
+#define del_singleshot_timer_sync(t) del_timer_sync(t)
+
extern void init_timers(void);
extern void run_local_timers(void);
extern void it_real_fn(unsigned long);