| Message ID | 20221130181325.1012760-12-paulmck@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | None | expand |
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 >
On Wed, Nov 30, 2022 at 10:13:21AM -0800, Paul E. McKenney 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> Acked-by: Tejun Heo <tj@kernel.org> Thanks.
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@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> > > Acked-by: Tejun Heo <tj@kernel.org> I applied both, thank you very much! Thanx, Paul
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)