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authorJoel Fernandes (Google) <joel@joelfernandes.org>2022-10-16 19:22:58 +0300
committerPaul E. McKenney <paulmck@kernel.org>2022-12-01 00:16:40 +0300
commit343a72e5e37d380b70534fae3acd7e5e39adb769 (patch)
tree86a0e3199612c1ff80602658c17ca699a875b3e0 /lib
parent54d87b0a0c19bc3f740e4cd4b87ba14ce2e4ea73 (diff)
downloadlinux-343a72e5e37d380b70534fae3acd7e5e39adb769.tar.xz
percpu-refcount: Use call_rcu_hurry() for atomic switch
Earlier commits in this series allow battery-powered systems to build their kernels with the default-disabled CONFIG_RCU_LAZY=y Kconfig option. This Kconfig option causes call_rcu() to delay its callbacks in order to batch callbacks. This means that a given RCU grace period covers more callbacks, thus reducing the number of grace periods, in turn reducing the amount of energy consumed, which increases battery lifetime which can be a very good thing. This is not a subtle effect: In some important use cases, the battery lifetime is increased by more than 10%. This CONFIG_RCU_LAZY=y option is available only for CPUs that offload callbacks, for example, CPUs mentioned in the rcu_nocbs kernel boot parameter passed to kernels built with CONFIG_RCU_NOCB_CPU=y. Delaying callbacks is normally not a problem because most callbacks do nothing but free memory. If the system is short on memory, a shrinker will kick all currently queued lazy callbacks out of their laziness, thus freeing their memory in short order. Similarly, the rcu_barrier() function, which blocks until all currently queued callbacks are invoked, will also kick lazy callbacks, thus enabling rcu_barrier() to complete in a timely manner. However, there are some cases where laziness is not a good option. For example, synchronize_rcu() invokes call_rcu(), and blocks until the newly queued callback is invoked. It would not be a good for synchronize_rcu() to block for ten seconds, even on an idle system. Therefore, synchronize_rcu() invokes call_rcu_hurry() instead of call_rcu(). The arrival of a non-lazy call_rcu_hurry() callback on a given CPU kicks any lazy callbacks that might be already queued on that CPU. After all, if there is going to be a grace period, all callbacks might as well get full benefit from it. Yes, this could be done the other way around by creating a call_rcu_lazy(), but earlier experience with this approach and feedback at the 2022 Linux Plumbers Conference shifted the approach to call_rcu() being lazy with call_rcu_hurry() for the few places where laziness is inappropriate. And another call_rcu() instance that cannot be lazy is the one on the percpu refcounter's "per-CPU to atomic switch" code path, which uses RCU when switching to atomic mode. The enqueued callback wakes up waiters waiting in the percpu_ref_switch_waitq. Allowing this callback to be lazy would result in unacceptable slowdowns for users of per-CPU refcounts, such as blk_pre_runtime_suspend(). Therefore, make __percpu_ref_switch_to_atomic() use call_rcu_hurry() in order to revert to the old behavior. [ paulmck: Apply s/call_rcu_flush/call_rcu_hurry/ feedback from Tejun Heo. ] Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Acked-by: Tejun Heo <tj@kernel.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Cc: Dennis Zhou <dennis@kernel.org> Cc: Christoph Lameter <cl@linux.com> Cc: <linux-mm@kvack.org>
Diffstat (limited to 'lib')
-rw-r--r--lib/percpu-refcount.c3
1 files changed, 2 insertions, 1 deletions
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index e5c5315da274..668f6aa6a75d 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -230,7 +230,8 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
percpu_ref_noop_confirm_switch;
percpu_ref_get(ref); /* put after confirmation */
- call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu);
+ call_rcu_hurry(&ref->data->rcu,
+ percpu_ref_switch_to_atomic_rcu);
}
static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)