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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 B3F39C73C42 for ; Tue, 9 Jul 2019 12:41:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8CAFE21537 for ; Tue, 9 Jul 2019 12:41:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726623AbfGIMli (ORCPT ); Tue, 9 Jul 2019 08:41:38 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:10776 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726010AbfGIMli (ORCPT ); Tue, 9 Jul 2019 08:41:38 -0400 Received: from pps.filterd (m0098421.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x69Cbjlh018060; Tue, 9 Jul 2019 08:41:04 -0400 Received: from pps.reinject (localhost [127.0.0.1]) by mx0a-001b2d01.pphosted.com with ESMTP id 2tmtq2159n-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 09 Jul 2019 08:41:04 -0400 Received: from m0098421.ppops.net (m0098421.ppops.net [127.0.0.1]) by pps.reinject (8.16.0.27/8.16.0.27) with SMTP id x69CbrTw018658; Tue, 9 Jul 2019 08:41:03 -0400 Received: from ppma03dal.us.ibm.com (b.bd.3ea9.ip4.static.sl-reverse.com [169.62.189.11]) by mx0a-001b2d01.pphosted.com with ESMTP id 2tmtq2158r-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 09 Jul 2019 08:41:03 -0400 Received: from pps.filterd (ppma03dal.us.ibm.com [127.0.0.1]) by ppma03dal.us.ibm.com (8.16.0.27/8.16.0.27) with SMTP id x69CdPgv010183; Tue, 9 Jul 2019 12:41:02 GMT Received: from b01cxnp22034.gho.pok.ibm.com (b01cxnp22034.gho.pok.ibm.com [9.57.198.24]) by ppma03dal.us.ibm.com with ESMTP id 2tjk96my6r-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 09 Jul 2019 12:41:02 +0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x69Cf1iL43974960 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 9 Jul 2019 12:41:01 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 5B9FCB2071; Tue, 9 Jul 2019 12:41:01 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2CF93B2064; Tue, 9 Jul 2019 12:41:01 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.26]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Tue, 9 Jul 2019 12:41:01 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 2EAD216C29D7; Tue, 9 Jul 2019 05:41:02 -0700 (PDT) Date: Tue, 9 Jul 2019 05:41:02 -0700 From: "Paul E. McKenney" To: Byungchul Park Cc: Joel Fernandes , josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@lge.com Subject: Re: [PATCH] rcu: Make jiffies_till_sched_qs writable Message-ID: <20190709124102.GR26519@linux.ibm.com> Reply-To: paulmck@linux.ibm.com References: <1562565609-12482-1-git-send-email-byungchul.park@lge.com> <20190708125013.GG26519@linux.ibm.com> <20190708130359.GA42888@google.com> <20190709055815.GA19459@X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190709055815.GA19459@X58A-UD3R> 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=2019-07-09_05:,, 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-1907090151 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 09, 2019 at 02:58:16PM +0900, Byungchul Park wrote: > On Mon, Jul 08, 2019 at 09:03:59AM -0400, Joel Fernandes wrote: > > > Actually, the intent was to only allow this to be changed at boot time. > > > Of course, if there is now a good reason to adjust it, it needs > > > to be adjustable. So what situation is making you want to change > > > jiffies_till_sched_qs at runtime? To what values is it proving useful > > > to adjust it? What (if any) relationships between this timeout and the > > > various other RCU timeouts need to be maintained? What changes to > > > rcutorture should be applied in order to test the ability to change > > > this at runtime? > > > > I am also interested in the context, are you changing it at runtime for > > experimentation? I recently was doing some performance experiments and it is > > quite interesting how reducing this value can shorten grace period times :) > > Hi Joel, > > I've read a thread talking about your experiment to see how the grace > periods change depending on the tunnable variables which was interesting > to me. While reading it, I found out jiffies_till_sched_qs is not > tunnable at runtime unlike jiffies_till_{first,next}_fqs which looks > like non-sense to me that's why I tried this patch. :) > > Hi Paul, > > IMHO, as much as we want to tune the time for fqs to be initiated, we > can also want to tune the time for the help from scheduler to start. > I thought only difference between them is a level of urgency. I might be > wrong. It would be appreciated if you let me know if I miss something. Hello, Byungchul, I understand that one hypothetically might want to tune this at runtime, but have you had need to tune this at runtime on a real production workload? If so, what problem was happening that caused you to want to do this tuning? > And it's ok even if the patch is turned down based on your criteria. :) If there is a real need, something needs to be provided to meet that need. But in the absence of a real need, past experience has shown that speculative tuning knobs usually do more harm than good. ;-) Hence my question to you about a real need. Thanx, Paul > Thanks, > Byungchul > > > Joel > > > > > > > Thanx, Paul > > > > > > > The function for setting jiffies_to_sched_qs, > > > > adjust_jiffies_till_sched_qs() will be called only if > > > > the value from sysfs != ULONG_MAX. And the value won't be adjusted > > > > unlike first/next fqs jiffies. > > > > > > > > While at it, changed the positions of two module_param()s downward. > > > > > > > > Signed-off-by: Byungchul Park > > > > --- > > > > kernel/rcu/tree.c | 22 ++++++++++++++++++++-- > > > > 1 file changed, 20 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > index a2f8ba2..a28e2fe 100644 > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -422,9 +422,7 @@ static int rcu_is_cpu_rrupt_from_idle(void) > > > > * quiescent-state help from rcu_note_context_switch(). > > > > */ > > > > static ulong jiffies_till_sched_qs = ULONG_MAX; > > > > -module_param(jiffies_till_sched_qs, ulong, 0444); > > > > static ulong jiffies_to_sched_qs; /* See adjust_jiffies_till_sched_qs(). */ > > > > -module_param(jiffies_to_sched_qs, ulong, 0444); /* Display only! */ > > > > > > > > /* > > > > * Make sure that we give the grace-period kthread time to detect any > > > > @@ -450,6 +448,18 @@ static void adjust_jiffies_till_sched_qs(void) > > > > WRITE_ONCE(jiffies_to_sched_qs, j); > > > > } > > > > > > > > +static int param_set_sched_qs_jiffies(const char *val, const struct kernel_param *kp) > > > > +{ > > > > + ulong j; > > > > + int ret = kstrtoul(val, 0, &j); > > > > + > > > > + if (!ret && j != ULONG_MAX) { > > > > + WRITE_ONCE(*(ulong *)kp->arg, j); > > > > + adjust_jiffies_till_sched_qs(); > > > > + } > > > > + return ret; > > > > +} > > > > + > > > > static int param_set_first_fqs_jiffies(const char *val, const struct kernel_param *kp) > > > > { > > > > ulong j; > > > > @@ -474,6 +484,11 @@ static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param > > > > return ret; > > > > } > > > > > > > > +static struct kernel_param_ops sched_qs_jiffies_ops = { > > > > + .set = param_set_sched_qs_jiffies, > > > > + .get = param_get_ulong, > > > > +}; > > > > + > > > > static struct kernel_param_ops first_fqs_jiffies_ops = { > > > > .set = param_set_first_fqs_jiffies, > > > > .get = param_get_ulong, > > > > @@ -484,8 +499,11 @@ static int param_set_next_fqs_jiffies(const char *val, const struct kernel_param > > > > .get = param_get_ulong, > > > > }; > > > > > > > > +module_param_cb(jiffies_till_sched_qs, &sched_qs_jiffies_ops, &jiffies_till_sched_qs, 0644); > > > > module_param_cb(jiffies_till_first_fqs, &first_fqs_jiffies_ops, &jiffies_till_first_fqs, 0644); > > > > module_param_cb(jiffies_till_next_fqs, &next_fqs_jiffies_ops, &jiffies_till_next_fqs, 0644); > > > > + > > > > +module_param(jiffies_to_sched_qs, ulong, 0444); /* Display only! */ > > > > module_param(rcu_kick_kthreads, bool, 0644); > > > > > > > > static void force_qs_rnp(int (*f)(struct rcu_data *rdp)); > > > > -- > > > > 1.9.1 > > > > > > > >