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McKenney" To: Joel Fernandes Cc: Byungchul Park , 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190708130359.GA42888@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19070813-0052-0000-0000-000003DBBFCC X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011395; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01229216; UDB=6.00647355; IPR=6.01010475; MB=3.00027632; MTD=3.00000008; XFM=3.00000015; UTC=2019-07-08 13:19:42 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19070813-0053-0000-0000-0000619C6ED2 Message-Id: <20190708131942.GH26519@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-07-08_04:,, 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-1907080167 Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org 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. ;-) 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. 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 > > > > > >