diff options
author | Tejun Heo <tj@kernel.org> | 2010-11-22 21:36:11 -0800 |
---|---|---|
committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2010-12-17 12:34:08 -0800 |
commit | e27fc9641e8ddc8146f8e01f06e5eba2469698de (patch) | |
tree | 5cff4c23cb113b901718d9c0f4a6e3081f3173bd /kernel/rcutree_plugin.h | |
parent | 46fdb0937f26124700fc9fc80da4776330cc00d3 (diff) |
rcu: increase synchronize_sched_expedited() batching
The fix in commit #6a0cc49 requires more than three concurrent instances
of synchronize_sched_expedited() before batching is possible. This
patch uses a ticket-counter-like approach that is also not unrelated to
Lai Jiangshan's Ring RCU to allow sharing of expedited grace periods even
when there are only two concurrent instances of synchronize_sched_expedited().
This commit builds on Tejun's original posting, which may be found at
http://lkml.org/lkml/2010/11/9/204, adding memory barriers, avoiding
overflow of signed integers (other than via atomic_t), and fixing the
detection of batching.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel/rcutree_plugin.h')
-rw-r--r-- | kernel/rcutree_plugin.h | 82 |
1 files changed, 62 insertions, 20 deletions
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h index c22c4ef2a0d..a3638710dc6 100644 --- a/kernel/rcutree_plugin.h +++ b/kernel/rcutree_plugin.h @@ -1025,7 +1025,8 @@ EXPORT_SYMBOL_GPL(synchronize_sched_expedited); #else /* #ifndef CONFIG_SMP */ -static atomic_t synchronize_sched_expedited_count = ATOMIC_INIT(0); +static atomic_t sync_sched_expedited_started = ATOMIC_INIT(0); +static atomic_t sync_sched_expedited_done = ATOMIC_INIT(0); static int synchronize_sched_expedited_cpu_stop(void *data) { @@ -1041,8 +1042,6 @@ static int synchronize_sched_expedited_cpu_stop(void *data) * robustness against future implementation changes. */ smp_mb(); /* See above comment block. */ - if (cpumask_first(cpu_online_mask) == smp_processor_id()) - atomic_inc(&synchronize_sched_expedited_count); return 0; } @@ -1056,43 +1055,86 @@ static int synchronize_sched_expedited_cpu_stop(void *data) * lock that is acquired by a CPU-hotplug notifier. Failing to * observe this restriction will result in deadlock. * - * The synchronize_sched_expedited_cpu_stop() function is called - * in stop-CPU context, but in order to keep overhead down to a dull - * roar, we don't force this function to wait for its counterparts - * on other CPUs. One instance of this function will increment the - * synchronize_sched_expedited_count variable per call to - * try_stop_cpus(), but there is no guarantee what order this instance - * will occur in. The worst case is that it is last on one call - * to try_stop_cpus(), and the first on the next call. This means - * that piggybacking requires that synchronize_sched_expedited_count - * be incremented by 3: this guarantees that the piggybacking - * task has waited through an entire cycle of context switches, - * even in the worst case. + * This implementation can be thought of as an application of ticket + * locking to RCU, with sync_sched_expedited_started and + * sync_sched_expedited_done taking on the roles of the halves + * of the ticket-lock word. Each task atomically increments + * sync_sched_expedited_started upon entry, snapshotting the old value, + * then attempts to stop all the CPUs. If this succeeds, then each + * CPU will have executed a context switch, resulting in an RCU-sched + * grace period. We are then done, so we use atomic_cmpxchg() to + * update sync_sched_expedited_done to match our snapshot -- but + * only if someone else has not already advanced past our snapshot. + * + * On the other hand, if try_stop_cpus() fails, we check the value + * of sync_sched_expedited_done. If it has advanced past our + * initial snapshot, then someone else must have forced a grace period + * some time after we took our snapshot. In this case, our work is + * done for us, and we can simply return. Otherwise, we try again, + * but keep our initial snapshot for purposes of checking for someone + * doing our work for us. + * + * If we fail too many times in a row, we fall back to synchronize_sched(). */ void synchronize_sched_expedited(void) { - int snap, trycount = 0; + int firstsnap, s, snap, trycount = 0; - smp_mb(); /* ensure prior mod happens before capturing snap. */ - snap = atomic_read(&synchronize_sched_expedited_count) + 2; + /* Note that atomic_inc_return() implies full memory barrier. */ + firstsnap = snap = atomic_inc_return(&sync_sched_expedited_started); get_online_cpus(); + + /* + * Each pass through the following loop attempts to force a + * context switch on each CPU. + */ while (try_stop_cpus(cpu_online_mask, synchronize_sched_expedited_cpu_stop, NULL) == -EAGAIN) { put_online_cpus(); + + /* No joy, try again later. Or just synchronize_sched(). */ if (trycount++ < 10) udelay(trycount * num_online_cpus()); else { synchronize_sched(); return; } - if (atomic_read(&synchronize_sched_expedited_count) - snap > 0) { + + /* Check to see if someone else did our work for us. */ + s = atomic_read(&sync_sched_expedited_done); + if (UINT_CMP_GE((unsigned)s, (unsigned)firstsnap)) { smp_mb(); /* ensure test happens before caller kfree */ return; } + + /* + * Refetching sync_sched_expedited_started allows later + * callers to piggyback on our grace period. We subtract + * 1 to get the same token that the last incrementer got. + * We retry after they started, so our grace period works + * for them, and they started after our first try, so their + * grace period works for us. + */ get_online_cpus(); + snap = atomic_read(&sync_sched_expedited_started) - 1; + smp_mb(); /* ensure read is before try_stop_cpus(). */ } - smp_mb__after_atomic_inc(); /* ensure post-GP actions seen after GP. */ + + /* + * Everyone up to our most recent fetch is covered by our grace + * period. Update the counter, but only if our work is still + * relevant -- which it won't be if someone who started later + * than we did beat us to the punch. + */ + do { + s = atomic_read(&sync_sched_expedited_done); + if (UINT_CMP_GE((unsigned)s, (unsigned)snap)) { + smp_mb(); /* ensure test happens before caller kfree */ + break; + } + } while (atomic_cmpxchg(&sync_sched_expedited_done, s, snap) != s); + put_online_cpus(); } EXPORT_SYMBOL_GPL(synchronize_sched_expedited); |