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author | 2010-04-01 17:37:01 -0700 | |
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committer | 2010-05-10 11:08:33 -0700 | |
commit | 25502a6c13745f4650cc59322bd198194f55e796 (patch) | |
tree | d76cc659d3ea797c5da4630e219ac363d17c44a6 /kernel/sched.c | |
parent | rcu: rename rcutiny rcu_ctrlblk to rcu_sched_ctrlblk (diff) | |
download | linux-25502a6c13745f4650cc59322bd198194f55e796.tar.gz linux-25502a6c13745f4650cc59322bd198194f55e796.tar.bz2 linux-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 'kernel/sched.c')
-rw-r--r-- | kernel/sched.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/sched.c b/kernel/sched.c index 3c2a54f70ffe..d8a213ccdc3b 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -3706,7 +3706,7 @@ need_resched: preempt_disable(); cpu = smp_processor_id(); rq = cpu_rq(cpu); - rcu_sched_qs(cpu); + rcu_note_context_switch(cpu); prev = rq->curr; switch_count = &prev->nivcsw; |