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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Sun, 1 Jul 2018 23:09:22 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22036.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w6239LMX1769886 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 2 Jul 2018 03:09:21 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2E8DBB205F; Sun, 1 Jul 2018 23:09:07 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id ED607B2064; Sun, 1 Jul 2018 23:09:06 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.206.224]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sun, 1 Jul 2018 23:09:06 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 944DE16CA314; Sun, 1 Jul 2018 20:11:32 -0700 (PDT) Date: Sun, 1 Jul 2018 20:11:32 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com Subject: Re: [PATCH RFC tip/core/rcu 1/2] rcu: Defer reporting RCU-preempt quiescent states when disabled Reply-To: paulmck@linux.vnet.ibm.com References: <20180627204835.GA25456@linux.vnet.ibm.com> <20180627204915.27253-1-paulmck@linux.vnet.ibm.com> <20180701183828.GB111992@joelaf.mtv.corp.google.com> <20180701222749.GD3593@linux.vnet.ibm.com> <20180702003553.GA95395@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180702003553.GA95395@joelaf.mtv.corp.google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18070203-0040-0000-0000-0000044812F0 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009292; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01055269; UDB=6.00541228; IPR=6.00833191; MB=3.00021956; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-02 03:09:26 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18070203-0041-0000-0000-0000084E2E3A Message-Id: <20180702031132.GI3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-01_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-1807020037 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jul 01, 2018 at 05:35:53PM -0700, Joel Fernandes wrote: > On Sun, Jul 01, 2018 at 03:27:49PM -0700, Paul E. McKenney wrote: > [...] > > > > +/* > > > > + * Report a deferred quiescent state if needed and safe to do so. > > > > + * As with rcu_preempt_need_deferred_qs(), "safe" involves only > > > > + * not being in an RCU read-side critical section. The caller must > > > > + * evaluate safety in terms of interrupt, softirq, and preemption > > > > + * disabling. > > > > + */ > > > > +static void rcu_preempt_deferred_qs(struct task_struct *t) > > > > +{ > > > > + unsigned long flags; > > > > + > > > > + if (!rcu_preempt_need_deferred_qs(t)) > > > > + return; > > > > + local_irq_save(flags); > > > > + rcu_preempt_deferred_qs_irqrestore(t, flags); > > > > +} > > > > + > > > > +/* > > > > + * Handle special cases during rcu_read_unlock(), such as needing to > > > > + * notify RCU core processing or task having blocked during the RCU > > > > + * read-side critical section. > > > > + */ > > > > +static void rcu_read_unlock_special(struct task_struct *t) > > > > +{ > > > > + unsigned long flags; > > > > + bool preempt_bh_were_disabled = !!(preempt_count() & ~HARDIRQ_MASK); > > > > + bool irqs_were_disabled; > > > > + > > > > + /* NMI handlers cannot block and cannot safely manipulate state. */ > > > > + if (in_nmi()) > > > > + return; > > > > + > > > > + local_irq_save(flags); > > > > + irqs_were_disabled = irqs_disabled_flags(flags); > > > > + if ((preempt_bh_were_disabled || irqs_were_disabled) && > > > > + t->rcu_read_unlock_special.b.blocked) { > > > > + /* Need to defer quiescent state until everything is enabled. */ > > > > + raise_softirq_irqoff(RCU_SOFTIRQ); > > > > + local_irq_restore(flags); > > > > + return; > > > > + } > > > > + rcu_preempt_deferred_qs_irqrestore(t, flags); > > > > +} > > > > + > > > > /* > > > > * Dump detailed information for all tasks blocking the current RCU > > > > * grace period on the specified rcu_node structure. > > > > @@ -737,10 +784,20 @@ static void rcu_preempt_check_callbacks(void) > > > > struct rcu_state *rsp = &rcu_preempt_state; > > > > struct task_struct *t = current; > > > > > > > > - if (t->rcu_read_lock_nesting == 0) { > > > > - rcu_preempt_qs(); > > > > + if (t->rcu_read_lock_nesting > 0 || > > > > + (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) { > > > > + /* No QS, force context switch if deferred. */ > > > > + if (rcu_preempt_need_deferred_qs(t)) > > > > + resched_cpu(smp_processor_id()); > > > > > > > > > Hi Paul, > > > > > > I had a similar idea of checking the preempt_count() sometime back but didn't > > > believe this path can be called with preempt enabled (for some reason ;-)). > > > Now that I've convinced myself that's possible, what do you think about > > > taking advantage of the opportunity to report a RCU-sched qs like below from > > > rcu_check_callbacks ? > > > > > > Did some basic testing, can roll into a patch later if you're Ok with it. > > > > The problem here is that the code patch above cannot be called > > with CONFIG_PREEMPT_COUNT=n, but the code below can. And if > > CONFIG_PREEMPT_COUNT=n, the return value from preempt_count() can be > > misleading. > > > > Or am I missing something here? > > That is true! so then I could also test if PREEMPT_RCU is enabled like you're > doing in the other path. > > thanks! > > ---8<----------------------- > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index fb440baf8ac6..03a460921dca 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2683,6 +2683,12 @@ void rcu_check_callbacks(int user) > rcu_note_voluntary_context_switch(current); > > } else if (!in_softirq()) { > + /* > + * Report RCU-sched qs if not in an RCU-sched read-side > + * critical section. > + */ > + if (IS_ENABLED(PREEMPT_RCU) && !(preempt_count() & PREEMPT_MASK)) For more precision, s/PREEMPT_RCU/CONFIG_PREEMPT_COUNT/ Hmmm... I recently queued a patch that redefines the RCU-bh update-side API in terms of the consolidated RCU implementation, so this "else" clause no longer exists. One approach would be to fold this condition (with the addition of SOFTIRQ_MASK) into the previous "if" condition, but that would call rcu_note_voluntary_context_switch() at bad times. So maybe this becomes a new "else if" clause. Another complication is an upcoming step that redefines the RCU-sched update-side API in terms of the consolidated RCU implementation, which will likely restructure this "if" statement yet again. So I will try to fold this idea in (with attribution). If I don't get it in place in a week or two, please remind me. Of course, one good way to remind me is to supply a patch against whatever this turns into. ;-) Thanx, Paul > + rcu_sched_qs(); > > /* > * Get here if this CPU did not take its interrupt from >