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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 9 Jul 2019 13:43:35 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x69ChYrK48759238 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 9 Jul 2019 12:43:34 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 89FD6B2066; Tue, 9 Jul 2019 12:43:34 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 5CCBDB2064; Tue, 9 Jul 2019 12:43:34 +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 12:43:34 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 61B0916C29D7; Tue, 9 Jul 2019 05:43:35 -0700 (PDT) Date: Tue, 9 Jul 2019 05:43:35 -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 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> <20190708131942.GH26519@linux.ibm.com> <20190709060558.GB19459@X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190709060558.GB19459@X58A-UD3R> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19070912-0072-0000-0000-00000446236D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011400; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01229684; UDB=6.00647637; IPR=6.01010943; MB=3.00027649; MTD=3.00000008; XFM=3.00000015; UTC=2019-07-09 12:43:38 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19070912-0073-0000-0000-00004CB665D8 Message-Id: <20190709124335.GS26519@linux.ibm.com> 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-1907090152 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 03:05:58PM +0900, Byungchul Park wrote: > On Mon, Jul 08, 2019 at 06:19:42AM -0700, Paul E. McKenney wrote: > > On Mon, Jul 08, 2019 at 09:03:59AM -0400, Joel Fernandes wrote: > > > Good morning! > > > > > > On Mon, Jul 08, 2019 at 05:50:13AM -0700, Paul E. McKenney wrote: > > > > On Mon, Jul 08, 2019 at 03:00:09PM +0900, Byungchul Park wrote: > > > > > jiffies_till_sched_qs is useless if it's readonly as it is used to set > > > > > jiffies_to_sched_qs with its value regardless of first/next fqs jiffies. > > > > > And it should be applied immediately on change through sysfs. > > > > > > It is interesting it can be setup at boot time, but not at runtime. I think > > > this can be mentioned in the change log that it is not really "read-only", > > > because it is something that can be dynamically changed as a kernel boot > > > parameter. > > > > In Byungchul's defense, the current module_param() permissions are > > 0444, which really is read-only. Although I do agree that they can > > be written at boot, one could use this same line of reasoning to argue > > that const variables can be written at compile time (or, for on-stack > > const variables, at function-invocation time). But we still call them > > "const". > > ;-) > > > > > 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 :) > > > > If you -really- want to reduce grace-period latencies, you can always > > boot with rcupdate.rcu_expedited=1. ;-) > > It's a quite different mechanism at the moment though... :( Quite true, but a rather effective mechanism for reducing grace-period latency, give or take the needs of aggressive real-time workloads. > > If you want to reduce grace-period latencies, but without all the IPIs > > that expedited grace periods give you, the rcutree.jiffies_till_first_fqs > > and rcutree.jiffies_till_next_fqs kernel boot parameters might be better > > places to start than rcutree.jiffies_till_sched_qs. For one thing, > > adjusting these two affects the value of jiffies_till_sched_qs. > > Do you mean: > > adjusting these two affects the value of *jiffies_to_sched_qs* (instead > of jiffies_till_sched_qs). > > Right? Yes, and apologies for my confusion! Thaxn, Paul > Thanks, > Byungchul > > > > > Thanx, Paul > > > > > 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 > > > > > > > > > > > > >