From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AEB1DC3A589 for ; Thu, 15 Aug 2019 21:45:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 78EEB206C1 for ; Thu, 15 Aug 2019 21:45:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732260AbfHOVpr (ORCPT ); Thu, 15 Aug 2019 17:45:47 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:49960 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732032AbfHOVpr (ORCPT ); Thu, 15 Aug 2019 17:45:47 -0400 Received: from pps.filterd (m0098399.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x7FLgCbk085275; Thu, 15 Aug 2019 17:45:44 -0400 Received: from ppma01dal.us.ibm.com (83.d6.3fa9.ip4.static.sl-reverse.com [169.63.214.131]) by mx0a-001b2d01.pphosted.com with ESMTP id 2udceqy29y-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 15 Aug 2019 17:45:43 -0400 Received: from pps.filterd (ppma01dal.us.ibm.com [127.0.0.1]) by ppma01dal.us.ibm.com (8.16.0.27/8.16.0.27) with SMTP id x7FLigLk030867; Thu, 15 Aug 2019 21:45:42 GMT Received: from b01cxnp22036.gho.pok.ibm.com (b01cxnp22036.gho.pok.ibm.com [9.57.198.26]) by ppma01dal.us.ibm.com with ESMTP id 2ucr3q7kbb-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 15 Aug 2019 21:45:42 +0000 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 x7FLjfcA11665988 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 15 Aug 2019 21:45:41 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D8614B2064; Thu, 15 Aug 2019 21:45:41 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id AB72AB205F; Thu, 15 Aug 2019 21:45:41 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.154]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 15 Aug 2019 21:45:41 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id A808016C1CD6; Thu, 15 Aug 2019 14:45:42 -0700 (PDT) Date: Thu, 15 Aug 2019 14:45:42 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: rcu@vger.kernel.org, frederic@kernel.org Subject: Re: need_heavy_qs flag for PREEMPT=y kernels Message-ID: <20190815214542.GM28441@linux.ibm.com> Reply-To: paulmck@linux.ibm.com References: <20190811211318.GX28441@linux.ibm.com> <20190812032142.GA171001@google.com> <20190812035306.GE28441@linux.ibm.com> <20190812212013.GB48751@google.com> <20190812230138.GS28441@linux.ibm.com> <20190813010249.GA129011@google.com> <20190815171714.GA1023@linux.ibm.com> <20190815200432.GB23194@google.com> <20190815203107.GL28441@linux.ibm.com> <20190815212216.GA224191@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190815212216.GA224191@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-15_09:,, 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=8.0.1-1906280000 definitions=main-1908150203 Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Thu, Aug 15, 2019 at 05:22:16PM -0400, Joel Fernandes wrote: > On Thu, Aug 15, 2019 at 01:31:07PM -0700, Paul E. McKenney wrote: > > On Thu, Aug 15, 2019 at 04:04:32PM -0400, Joel Fernandes wrote: > > > On Thu, Aug 15, 2019 at 10:17:14AM -0700, Paul E. McKenney wrote: > > > > On Mon, Aug 12, 2019 at 09:02:49PM -0400, Joel Fernandes wrote: > > > > > On Mon, Aug 12, 2019 at 04:01:38PM -0700, Paul E. McKenney wrote: > > > > > > > > [ . . . ] > > > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > > > index 8c494a692728..ad906d6a74fb 100644 > > > > > > --- a/kernel/rcu/tree.c > > > > > > +++ b/kernel/rcu/tree.c > > > > > > @@ -651,6 +651,12 @@ static __always_inline void rcu_nmi_exit_common(bool irq) > > > > > > */ > > > > > > if (rdp->dynticks_nmi_nesting != 1) { > > > > > > trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, rdp->dynticks); > > > > > > + if (tick_nohz_full_cpu(rdp->cpu) && > > > > > > + rdp->dynticks_nmi_nesting == 2 && > > > > > > + rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) { > > > > > > + rdp->rcu_forced_tick = true; > > > > > > + tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU); > > > > > > + } > > > > > > > > > > > > > > > Instead of checking dynticks_nmi_nesting == 2 in rcu_nmi_exit_common(), can > > > > > we do the tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU) from > > > > > rcu_nmi_enter_common() ? We could add this code there, under the "if > > > > > (rcu_dynticks_curr_cpu_in_eqs())". > > > > > > > > This would need to go in an "else" clause, correct? But there would still > > > > want to be a check for interrupt from base level (which would admittedly > > > > be an equality comparison with zero) and we would also still need to check > > > > for rdp->rcu_urgent_qs && !rdp->rcu_forced_tick. > > > > > > True, agreed. I replied to this before saying it should be > > > !rcu_dynticks_curr_cpu_in_eqs() in the "if" ;) But it seems I could also be > > > missing the check for TICK_DEP_MASK_RCU in my tree so I think we need this as > > > well which is below as diff. Testing it more now! > > > > > > And, with this I do get many more ticks during the test. But there are > > > intervals where the tick is not seen. Still it is much better than before: > > > > > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > > > index be9707f68024..e697c7a2ce67 100644 > > > --- a/kernel/time/tick-sched.c > > > +++ b/kernel/time/tick-sched.c > > > @@ -198,6 +198,10 @@ static bool check_tick_dependency(atomic_t *dep) > > > return true; > > > } > > > > > > + if (val & TICK_DEP_MASK_CLOCK_RCU) { > > > + return true; > > > + } > > > + > > > return false; > > > } > > > > That one is not in my tree, either. Frederic, should I add this to > > your patch? For that matter, may I add your Signed-off-by as well? > > Your original is in my -rcu tree at: > > > > 0cb41806c799 ("EXP nohz: Add TICK_DEP_BIT_RCU") > > > > I am testing Joel's suggested addition now. > > Actually there's more addition needed! I found another thing missing: > > There's a per-cpu &tick_dep_mask and a per-cpu ts->tick_dep_mask. It seems > RCU is setting the latter. As I understand it, tick_dep_mask forces the tick on globally, ts->tick_dep_mask forces it on for a specific CPU (which RCU uses when it needs a quiescent state from that CPU), current->tick_dep_mask forces it on for a specific task (which RCU uses for callback invocation and certain rcutorture kthreads), and I don't pretend to understand current->signal->tick_dep_mask (the comment says something about POSIX CPU timers). But it looks to me that can_stop_full_tick() and check_tick_dependency() already cover all of these. What am I missing? > So I added a check for both, below is the diff: > > However, I see in some cases that the tick_dep_mask is just 0 but I have to > debug that tomorrow if that's an issue on the RCU side of things. For now, > below should be the completed Frederick patch which you could squash into his > if he's Ok with it: > > ---8<----------------------- > > diff --git a/include/linux/tick.h b/include/linux/tick.h > index f92a10b5e112..3f476e2a4bf7 100644 > --- a/include/linux/tick.h > +++ b/include/linux/tick.h > @@ -108,7 +108,8 @@ enum tick_dep_bits { > TICK_DEP_BIT_POSIX_TIMER = 0, > TICK_DEP_BIT_PERF_EVENTS = 1, > TICK_DEP_BIT_SCHED = 2, > - TICK_DEP_BIT_CLOCK_UNSTABLE = 3 > + TICK_DEP_BIT_CLOCK_UNSTABLE = 3, > + TICK_DEP_BIT_RCU = 4 > }; > > #define TICK_DEP_MASK_NONE 0 > @@ -116,6 +117,7 @@ enum tick_dep_bits { > #define TICK_DEP_MASK_PERF_EVENTS (1 << TICK_DEP_BIT_PERF_EVENTS) > #define TICK_DEP_MASK_SCHED (1 << TICK_DEP_BIT_SCHED) > #define TICK_DEP_MASK_CLOCK_UNSTABLE (1 << TICK_DEP_BIT_CLOCK_UNSTABLE) > +#define TICK_DEP_MASK_RCU (1 << TICK_DEP_BIT_RCU) > > #ifdef CONFIG_NO_HZ_COMMON > extern bool tick_nohz_enabled; > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index be9707f68024..a613916cc3f0 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -198,6 +198,11 @@ static bool check_tick_dependency(atomic_t *dep) > return true; > } > > + if (val & TICK_DEP_MASK_RCU) { > + trace_tick_stop(0, TICK_DEP_MASK_RCU); > + return true; > + } > + > return false; > } > > @@ -208,8 +213,13 @@ static bool can_stop_full_tick(int cpu, struct tick_sched *ts) > if (unlikely(!cpu_online(cpu))) > return false; > > - if (check_tick_dependency(&tick_dep_mask)) > + if (check_tick_dependency(&ts->tick_dep_mask)) { > return false; > + } > + > + if (check_tick_dependency(&tick_dep_mask)) { > + return false; > + } > > if (check_tick_dependency(&ts->tick_dep_mask)) > return false; You lost me on this one. How does it help to check ts->tick_dep_mask twice? And why is it important to check it before checking tick_dep_mask, especially given that the common case of all-zero masks will cause all to be checked anyway? Thanx, Paul