Message ID | 20221130181325.1012760-13-paulmck@kernel.org |
---|---|
State | New |
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McKenney" <paulmck@kernel.org> To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, Uladzislau Rezki <urezki@gmail.com>, Joel Fernandes <joel@joelfernandes.org>, Tejun Heo <tj@kernel.org>, Lai Jiangshan <jiangshanlai@gmail.com>, "Paul E . McKenney" <paulmck@kernel.org> Subject: [PATCH rcu 13/16] workqueue: Make queue_rcu_work() use call_rcu_hurry() Date: Wed, 30 Nov 2022 10:13:22 -0800 Message-Id: <20221130181325.1012760-13-paulmck@kernel.org> X-Mailer: git-send-email 2.31.1.189.g2e36527f23 In-Reply-To: <20221130181316.GA1012431@paulmck-ThinkPad-P17-Gen-1> References: <20221130181316.GA1012431@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-7.0 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,RCVD_IN_DNSWL_HI,SPF_HELO_NONE, SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: <linux-kernel.vger.kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1750946028109317499?= X-GMAIL-MSGID: =?utf-8?q?1750946028109317499?= |
Series |
Lazy call_rcu() updates for v6.2
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Commit Message
Paul E. McKenney
Nov. 30, 2022, 6:13 p.m. UTC
From: Uladzislau Rezki <urezki@gmail.com> 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 <urezki@gmail.com> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Cc: Tejun Heo <tj@kernel.org> Cc: Lai Jiangshan <jiangshanlai@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> --- kernel/workqueue.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
Comments
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 <paulmck@kernel.org> wrote: > > From: Uladzislau Rezki <urezki@gmail.com> > > 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 <urezki@gmail.com> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > Cc: Tejun Heo <tj@kernel.org> > Cc: Lai Jiangshan <jiangshanlai@gmail.com> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > --- > 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 >
On Wed, Nov 30, 2022 at 10:13:22AM -0800, Paul E. McKenney wrote: > From: Uladzislau Rezki <urezki@gmail.com> > > 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 <urezki@gmail.com> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > Cc: Tejun Heo <tj@kernel.org> > Cc: Lai Jiangshan <jiangshanlai@gmail.com> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Acked-by: Tejun Heo <tj@kernel.org> Thanks.
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; }