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| author | Paul E. McKenney <paul.mckenney@linaro.org> | 2012-05-10 16:41:44 -0700 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2012-06-06 20:43:28 -0700 |
| commit | aa9b16306e3243229580ff889cc59fd66bf77973 (patch) | |
| tree | f01812ee99804cd7d7533a1d3cba1e9d439e6f63 /include | |
| parent | 5955f7eecd77d6b440db278b266cfecdb72ecd00 (diff) | |
| download | linux-aa9b16306e3243229580ff889cc59fd66bf77973.tar.gz linux-aa9b16306e3243229580ff889cc59fd66bf77973.tar.bz2 linux-aa9b16306e3243229580ff889cc59fd66bf77973.zip | |
rcu: Precompute RCU_FAST_NO_HZ timer offsets
When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick()
calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the
next wakeup time based on the timer wheels. Only later, when actually
entering the idle loop, rcu_prepare_for_idle() will be invoked. In some
cases, rcu_prepare_for_idle() will post timers to wake the CPU back up.
But all for naught: The next wakeup time for the CPU has already been
computed, and posting a timer afterwards does not force that wakeup
time to be recomputed. This means that rcu_prepare_for_idle()'s have
no effect.
This is not a problem on a busy system because something else will wake
up the CPU soon enough. However, on lightly loaded systems, the CPU
might stay asleep for a considerable length of time. If that CPU has
a callback that the rest of the system is waiting on, the system might
run very slowly or (in theory) even hang.
This commit avoids this problem by having rcu_needs_cpu() give
tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU
to wake back up, which tick_nohz_stop_sched_tick() takes into account
when programming the CPU's wakeup time. An alternative approach is
for rcu_prepare_for_idle() to use hrtimers instead of normal timers,
but timers are much more efficient than are hrtimers for frequently
and repeatedly posting and cancelling a given timer, which is exactly
what RCU_FAST_NO_HZ does.
Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
Diffstat (limited to 'include')
| -rw-r--r-- | include/linux/rcutiny.h | 6 | ||||
| -rw-r--r-- | include/linux/rcutree.h | 2 |
2 files changed, 5 insertions, 3 deletions
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h index adb5e5a38cae..854dc4c5c271 100644 --- a/include/linux/rcutiny.h +++ b/include/linux/rcutiny.h @@ -87,8 +87,9 @@ static inline void kfree_call_rcu(struct rcu_head *head, #ifdef CONFIG_TINY_RCU -static inline int rcu_needs_cpu(int cpu) +static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies) { + *delta_jiffies = ULONG_MAX; return 0; } @@ -96,8 +97,9 @@ static inline int rcu_needs_cpu(int cpu) int rcu_preempt_needs_cpu(void); -static inline int rcu_needs_cpu(int cpu) +static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies) { + *delta_jiffies = ULONG_MAX; return rcu_preempt_needs_cpu(); } diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h index 3c6083cde4fc..952b79339304 100644 --- a/include/linux/rcutree.h +++ b/include/linux/rcutree.h @@ -32,7 +32,7 @@ extern void rcu_init(void); extern void rcu_note_context_switch(int cpu); -extern int rcu_needs_cpu(int cpu); +extern int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies); extern void rcu_cpu_stall_reset(void); /* |