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[209.132.180.67]) by mx.google.com with ESMTP id c8si19133893pls.436.2019.07.08.23.07.20; Mon, 08 Jul 2019 23:07:36 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726760AbfGIGGz (ORCPT + 99 others); Tue, 9 Jul 2019 02:06:55 -0400 Received: from lgeamrelo13.lge.com ([156.147.23.53]:51025 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725886AbfGIGGy (ORCPT ); Tue, 9 Jul 2019 02:06:54 -0400 Received: from unknown (HELO lgemrelse7q.lge.com) (156.147.1.151) by 156.147.23.53 with ESMTP; 9 Jul 2019 15:06:51 +0900 X-Original-SENDERIP: 156.147.1.151 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO X58A-UD3R) (10.177.222.33) by 156.147.1.151 with ESMTP; 9 Jul 2019 15:06:51 +0900 X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Tue, 9 Jul 2019 15:05:58 +0900 From: Byungchul Park To: "Paul E. McKenney" 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: <20190709060558.GB19459@X58A-UD3R> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190708131942.GH26519@linux.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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... :( > 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? 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 > > > > > > > > >