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

[Joel Fernandes <joel@joelfernandes.org>]

Hi Tejun,

Could you give your ACK for this patch, for percpu refcount? The API
is renamed like in the workqueue one, as well.

Thanks a lot,

- Joel


On Wed, Nov 30, 2022 at 6:13 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
>
> 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>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Dennis Zhou <dennis@kernel.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: <linux-mm@kvack.org>
> ---
>  lib/percpu-refcount.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
> index e5c5315da2741..668f6aa6a75de 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)
> --
> 2.31.1.189.g2e36527f23
>