diff options
author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2010-04-01 17:37:01 -0700 |
---|---|---|
committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2010-05-10 11:08:33 -0700 |
commit | 25502a6c13745f4650cc59322bd198194f55e796 (patch) | |
tree | d76cc659d3ea797c5da4630e219ac363d17c44a6 /include/linux/rcutiny.h | |
parent | 99652b54de1ee094236f7171485214071af4ef31 (diff) | |
download | linux-stable-25502a6c13745f4650cc59322bd198194f55e796.tar.gz linux-stable-25502a6c13745f4650cc59322bd198194f55e796.tar.bz2 linux-stable-25502a6c13745f4650cc59322bd198194f55e796.zip |
rcu: refactor RCU's context-switch handling
The addition of preemptible RCU to treercu resulted in a bit of
confusion and inefficiency surrounding the handling of context switches
for RCU-sched and for RCU-preempt. For RCU-sched, a context switch
is a quiescent state, pure and simple, just like it always has been.
For RCU-preempt, a context switch is in no way a quiescent state, but
special handling is required when a task blocks in an RCU read-side
critical section.
However, the callout from the scheduler and the outer loop in ksoftirqd
still calls something named rcu_sched_qs(), whose name is no longer
accurate. Furthermore, when rcu_check_callbacks() notes an RCU-sched
quiescent state, it ends up unnecessarily (though harmlessly, aside
from the performance hit) enqueuing the current task if it happens to
be running in an RCU-preempt read-side critical section. This not only
increases the maximum latency of scheduler_tick(), it also needlessly
increases the overhead of the next outermost rcu_read_unlock() invocation.
This patch addresses this situation by separating the notion of RCU's
context-switch handling from that of RCU-sched's quiescent states.
The context-switch handling is covered by rcu_note_context_switch() in
general and by rcu_preempt_note_context_switch() for preemptible RCU.
This permits rcu_sched_qs() to handle quiescent states and only quiescent
states. It also reduces the maximum latency of scheduler_tick(), though
probably by much less than a microsecond. Finally, it means that tasks
within preemptible-RCU read-side critical sections avoid incurring the
overhead of queuing unless there really is a context switch.
Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Diffstat (limited to 'include/linux/rcutiny.h')
-rw-r--r-- | include/linux/rcutiny.h | 4 |
1 files changed, 4 insertions, 0 deletions
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h index bbeb55b7709b..ff22b97fb979 100644 --- a/include/linux/rcutiny.h +++ b/include/linux/rcutiny.h @@ -29,6 +29,10 @@ void rcu_sched_qs(int cpu); void rcu_bh_qs(int cpu); +static inline void rcu_note_context_switch(int cpu) +{ + rcu_sched_qs(cpu); +} #define __rcu_read_lock() preempt_disable() #define __rcu_read_unlock() preempt_enable() |