From a7e30c0e9a5f95b7f74e6272d9c75fd65c897721 Mon Sep 17 00:00:00 2001 From: Uladzislau Rezki Date: Sun, 16 Oct 2022 16:23:03 +0000 Subject: [PATCH] workqueue: Make queue_rcu_work() use call_rcu_hurry() 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 them. 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 in queue_rcu_work(), given that callers to queue_rcu_work() are not necessarily OK with long delays. Therefore, make queue_rcu_work() 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: Uladzislau Rezki Signed-off-by: Joel Fernandes (Google) Acked-by: Tejun Heo Cc: Lai Jiangshan Signed-off-by: Paul E. McKenney --- kernel/workqueue.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/workqueue.c b/kernel/workqueue.c index 7cd5f5e7e0a1..07895deca271 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -1771,7 +1771,7 @@ bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork) if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) { rwork->wq = wq; - call_rcu(&rwork->rcu, rcu_work_rcufn); + call_rcu_hurry(&rwork->rcu, rcu_work_rcufn); return true; } -- 2.25.1