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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Sun, 24 Mar 2019 23:41:16 -0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22035.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x2ONfFwo18415816 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Sun, 24 Mar 2019 23:41:15 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6346FB205F; Sun, 24 Mar 2019 23:41:15 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3595DB2064; Sun, 24 Mar 2019 23:41:15 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.188]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sun, 24 Mar 2019 23:41:15 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 86F1E16C2DE1; Sun, 24 Mar 2019 16:42:11 -0700 (PDT) Date: Sun, 24 Mar 2019 16:42:11 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , tglx@linutronix.de, Mike Galbraith Subject: Re: [PATCH v3] rcu: Allow to eliminate softirq processing from rcutree Reply-To: paulmck@linux.ibm.com References: <20190320161500.GK4102@linux.ibm.com> <20190320163532.mr32oi53iaueuizw@linutronix.de> <20190320173001.GM4102@linux.ibm.com> <20190320175952.yh6yfy64vaiurszw@linutronix.de> <20190320181210.GO4102@linux.ibm.com> <20190320181435.x3qyutwqllmq5zbk@linutronix.de> <20190320211333.eq7pwxnte7la67ph@linutronix.de> <20190322234819.GA99360@google.com> <20190323002519.GV4102@linux.ibm.com> <20190323161002.GA17112@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190323161002.GA17112@linux.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19032423-0068-0000-0000-000003AA6BB3 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010809; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000282; SDB=6.01179187; UDB=6.00617002; IPR=6.00959897; MB=3.00026141; MTD=3.00000008; XFM=3.00000015; UTC=2019-03-24 23:41:18 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19032423-0069-0000-0000-000047ECBB2E Message-Id: <20190324234211.GA28064@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-03-24_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1903240181 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Mar 23, 2019 at 09:10:02AM -0700, Paul E. McKenney wrote: > On Fri, Mar 22, 2019 at 05:25:19PM -0700, Paul E. McKenney wrote: > > On Fri, Mar 22, 2019 at 07:48:19PM -0400, Joel Fernandes wrote: > > > On Wed, Mar 20, 2019 at 10:13:33PM +0100, Sebastian Andrzej Siewior wrote: > > > > Running RCU out of softirq is a problem for some workloads that would > > > > like to manage RCU core processing independently of other softirq > > > > work, for example, setting kthread priority. This commit therefore > > > > introduces the `rcunosoftirq' option which moves the RCU core work > > > > from softirq to a per-CPU/per-flavor SCHED_OTHER kthread named rcuc. > > > > The SCHED_OTHER approach avoids the scalability problems that appeared > > > > with the earlier attempt to move RCU core processing to from softirq > > > > to kthreads. That said, kernels built with RCU_BOOST=y will run the > > > > rcuc kthreads at the RCU-boosting priority. > > > [snip] > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > index 0f31b79eb6761..05a1e42fdaf10 100644 > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -51,6 +51,12 @@ > > > > #include > > > > #include > > > > #include > > > > +#include > > > > +#include > > > > +#include > > > > +#include > > > > +#include > > > > +#include "../time/tick-internal.h" > > > > > > > > #include "tree.h" > > > > #include "rcu.h" > > > > @@ -92,6 +98,9 @@ struct rcu_state rcu_state = { > > > > /* Dump rcu_node combining tree at boot to verify correct setup. */ > > > > static bool dump_tree; > > > > module_param(dump_tree, bool, 0444); > > > > +/* Move RCU_SOFTIRQ to rcuc kthreads. */ > > > > +static bool use_softirq = 1; > > > > +module_param(use_softirq, bool, 0444); > > > > /* Control rcu_node-tree auto-balancing at boot time. */ > > > > static bool rcu_fanout_exact; > > > > module_param(rcu_fanout_exact, bool, 0444); > > > > @@ -2253,7 +2262,7 @@ void rcu_force_quiescent_state(void) > > > > EXPORT_SYMBOL_GPL(rcu_force_quiescent_state); > > > > > > > > /* Perform RCU core processing work for the current CPU. */ > > > > -static __latent_entropy void rcu_core(struct softirq_action *unused) > > > > +static __latent_entropy void rcu_core(void) > > > > { > > > > unsigned long flags; > > > > struct rcu_data *rdp = raw_cpu_ptr(&rcu_data); > > > > @@ -2295,6 +2304,34 @@ static __latent_entropy void rcu_core(struct softirq_action *unused) > > > > trace_rcu_utilization(TPS("End RCU core")); > > > > } > > > > > > > > +static void rcu_core_si(struct softirq_action *h) > > > > +{ > > > > + rcu_core(); > > > > +} > > > > + > > > > +static void rcu_wake_cond(struct task_struct *t, int status) > > > > +{ > > > > + /* > > > > + * If the thread is yielding, only wake it when this > > > > + * is invoked from idle > > > > + */ > > > > + if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current))) > > > > + wake_up_process(t); > > > > +} > > > > + > > > > +static void invoke_rcu_core_kthread(void) > > > > +{ > > > > + struct task_struct *t; > > > > + unsigned long flags; > > > > + > > > > + local_irq_save(flags); > > > > + __this_cpu_write(rcu_data.rcu_cpu_has_work, 1); > > > > + t = __this_cpu_read(rcu_data.rcu_cpu_kthread_task); > > > > + if (t != NULL && t != current) > > > > + rcu_wake_cond(t, __this_cpu_read(rcu_data.rcu_cpu_kthread_status)); > > > > + local_irq_restore(flags); > > > > +} > > > > + > > > > /* > > > > * Schedule RCU callback invocation. If the running implementation of RCU > > > > * does not support RCU priority boosting, just do a direct call, otherwise > > > > @@ -2306,19 +2343,95 @@ static void invoke_rcu_callbacks(struct rcu_data *rdp) > > > > { > > > > if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > > > > return; > > > > - if (likely(!rcu_state.boost)) { > > > > - rcu_do_batch(rdp); > > > > - return; > > > > - } > > > > - invoke_rcu_callbacks_kthread(); > > > > + if (rcu_state.boost || !use_softirq) > > > > + invoke_rcu_core_kthread(); > > > > + rcu_do_batch(rdp); > > > > > > Shouldn't there be an else before the rcu_do_batch? If we are waking up the > > > rcuc thread, then that will do the rcu_do_batch when it runs right? > > > > > > Something like: > > > if (rcu_state.boost || !use_softirq) > > > invoke_rcu_core_kthread(); > > > else > > > rcu_do_batch(rdp); > > > > > > Previous code similarly had a return; also. > > > > I believe that you are correct, so I will give it a shot. Good eyes! > > Yet rcutorture disagrees. Actually, if we are using rcuc kthreads, this > is only ever invoked from within tha thread, so the only check we need is > for the scheduler being operational. I am therefore trying this one out. > > Thoughts? And rcutorture likes this one, though at this point this function should be pulled into its sole callsite. ;-) Thanx, Paul > ------------------------------------------------------------------------ > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 76d6c0902f66..8d6ebc0944ec 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2333,18 +2333,16 @@ static void invoke_rcu_core_kthread(void) > } > > /* > - * Schedule RCU callback invocation. If the running implementation of RCU > - * does not support RCU priority boosting, just do a direct call, otherwise > - * wake up the per-CPU kernel kthread. Note that because we are running > - * on the current CPU with softirqs disabled, the rcu_cpu_kthread_task > - * cannot disappear out from under us. > + * Do RCU callback invocation. Not that if we are running !use_softirq, > + * we are already in the rcuc kthread. If callbacks are offloaded, then > + * ->cblist is always empty, so we don't get here. Therefore, we only > + * ever need to check for the scheduler being operational (some callbacks > + * do wakeups, so we do need the scheduler). > */ > static void invoke_rcu_callbacks(struct rcu_data *rdp) > { > if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > return; > - if (rcu_state.boost || !use_softirq) > - invoke_rcu_core_kthread(); > rcu_do_batch(rdp); > } >