On Wed, Nov 30, 2022 at 09:43:44AM -1000, Tejun Heo wrote: > On Wed, Nov 30, 2022 at 10:13:21AM -0800, Paul E. McKenney wrote: > > From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> > > > > 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@xxxxxxxxxxxxxxxxx> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > Cc: Dennis Zhou <dennis@xxxxxxxxxx> > > Cc: Tejun Heo <tj@xxxxxxxxxx> > > Cc: Christoph Lameter <cl@xxxxxxxxx> > > Cc: <linux-mm@xxxxxxxxx> > > Acked-by: Tejun Heo <tj@xxxxxxxxxx> I applied both, thank you very much! Thanx, Paul
