Re: [PATCH rcu 12/16] percpu-refcount: Use call_rcu_hurry() for atomic switch

[Paul E. McKenney]

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