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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 9 Jul 2018 17:03:15 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w69L3EGF14418272 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 9 Jul 2018 21:03:14 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D3536B2065; Mon, 9 Jul 2018 17:02:48 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id AD863B2064; Mon, 9 Jul 2018 17:02:48 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 9 Jul 2018 17:02:48 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 8D94116C2E19; Mon, 9 Jul 2018 14:05:32 -0700 (PDT) Date: Mon, 9 Jul 2018 14:05:32 -0700 From: "Paul E. McKenney" To: David Woodhouse Cc: Peter Zijlstra , mhillenb@amazon.de, linux-kernel Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested Reply-To: paulmck@linux.vnet.ibm.com References: <20180709125516.GO2476@hirez.programming.kicks-ass.net> <20180709130227.GQ2512@hirez.programming.kicks-ass.net> <20180709142932.GO3593@linux.vnet.ibm.com> <20180709152632.GX2476@hirez.programming.kicks-ass.net> <20180709163432.GV3593@linux.vnet.ibm.com> <1531162254.26547.3.camel@infradead.org> <20180709203441.GE3593@linux.vnet.ibm.com> <1531168538.26547.5.camel@infradead.org> <20180709204248.GF3593@linux.vnet.ibm.com> <1531169145.26547.8.camel@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1531169145.26547.8.camel@infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18070921-0068-0000-0000-0000031587AA X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009341; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01058987; UDB=6.00543460; IPR=6.00836907; MB=3.00022076; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-09 21:03:16 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18070921-0069-0000-0000-000044F8EA18 Message-Id: <20180709210532.GH3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-09_07:,, 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=8.0.1-1806210000 definitions=main-1807090237 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 09, 2018 at 09:45:45PM +0100, David Woodhouse wrote: > On Mon, 2018-07-09 at 13:42 -0700, Paul E. McKenney wrote: > > On Mon, Jul 09, 2018 at 09:35:38PM +0100, David Woodhouse wrote: > > >  > > >  > > > On Mon, 2018-07-09 at 13:34 -0700, Paul E. McKenney wrote: > > > >  > > > > So here are the possible code paths when .rcu_urgent_qs is set to true: > > > >  > > > > 1.      A context switch will record the quiescent state and clear > > > >         .rcu_urgent_qs.  (The failure to do the clearing in current -rcu > > > >         for PREEMPT builds is a performance bug that I need to fix.) > > >  > > > What if there's nothing else runnable and there is no actual context > > > switch? > > > > The scheduler invokes rcu_note_context_switch() before looking to see > > if there really will or won't be a context switch. > > > > I am sure that Peter will correct me if I am confused on this point.  ;-) > > Ah, OK. Yes, that looks correct. Thanks. Here is hoping! > I'll give your patch a spin tomorrow, unless Marius beats me to it. Please see below for the version that I eventually queued. Should Marius have a Reported-by? If so, please tell me his full name so I can add that. Thanx, Paul ------------------------------------------------------------------------ commit e1e91fd0796dc544a77ddfaa5afbfc1f5fb42ecd Author: Paul E. McKenney Date: Mon Jul 9 13:47:30 2018 -0700 rcu: Make need_resched() respond to urgent RCU-QS needs The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent need for an RCU quiescent state from the force-quiescent-state processing within the grace-period kthread to context switches and to cond_resched(). Unfortunately, such urgent needs are not communicated to need_resched(), which is sometimes used to decide when to invoke cond_resched(), for but one example, within the KVM vcpu_run() function. As of v4.15, this can result in synchronize_sched() being delayed by up to ten seconds, which can be problematic, to say nothing of annoying. This commit therefore checks rcu_dynticks.rcu_urgent_qs from within rcu_check_callbacks(), which is invoked from the scheduling-clock interrupt handler. If the current task is not an idle task and is not executing in usermode, a context switch is forced, and either way, the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current task is an idle task, then RCU's dyntick-idle code will detect the quiescent state, so no further action is required. Similarly, if the task is executing in usermode, other code in rcu_check_callbacks() and its called functions will report the corresponding quiescent state. Reported-by: David Woodhouse Suggested-by: Peter Zijlstra Signed-off-by: Paul E. McKenney diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 51919985f6cf..ea756bb64eb3 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2496,6 +2496,15 @@ void rcu_check_callbacks(int user) { trace_rcu_utilization(TPS("Start scheduler-tick")); raw_cpu_inc(rcu_data.ticks_this_gp); + /* The load-acquire pairs with the store-release setting to true. */ + if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) { + /* Idle and userspace execution already are quiescent states. */ + if (!is_idle_task(current) && !user) { + set_tsk_need_resched(current); + set_preempt_need_resched(); + } + __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false); + } rcu_flavor_check_callbacks(user); if (rcu_pending()) invoke_rcu_core();