[13/16] workqueue: Make queue_rcu_work() use call_rcu_hurry()

Commit Message

Paul E. McKenney Nov. 30, 2022, 6:13 p.m. UTC

From: Uladzislau Rezki <[e-mail]>

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 <[e-mail]>
Signed-off-by: Joel Fernandes (Google) <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Lai Jiangshan <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
---
 kernel/workqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Joel Fernandes Nov. 30, 2022, 6:18 p.m. UTC | #1

Hi Tejun,

The API is renamed to call_rcu_hurry() as you and Paul discussed, to
avoid conflicts with the word flush. Could you give your ACK for this
patch, for workqueue?

Thanks a lot,

- Joel

On Wed, Nov 30, 2022 at 6:13 PM Paul E. McKenney <[e-mail]> wrote:
>
> From: Uladzislau Rezki <[e-mail]>
>
> 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 <[e-mail]>
> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> Cc: Tejun Heo <[e-mail]>
> Cc: Lai Jiangshan <[e-mail]>
> Signed-off-by: Paul E. McKenney <[e-mail]>
> ---
>  kernel/workqueue.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 7cd5f5e7e0a1b..07895deca2711 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;
>         }
>
> --
> 2.31.1.189.g2e36527f23
>

Tejun Heo Nov. 30, 2022, 7:43 p.m. UTC | #2

On Wed, Nov 30, 2022 at 10:13:22AM -0800, Paul E. McKenney wrote:
> From: Uladzislau Rezki <[e-mail]>
> 
> 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 <[e-mail]>
> Signed-off-by: Joel Fernandes (Google) <[e-mail]>
> Cc: Tejun Heo <[e-mail]>
> Cc: Lai Jiangshan <[e-mail]>
> Signed-off-by: Paul E. McKenney <[e-mail]>

Acked-by: Tejun Heo <[e-mail]>

Thanks.

Patch

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 7cd5f5e7e0a1b..07895deca2711 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;
 	}