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> Thanks. -- tejun
