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Date:   Mon, 8 Jul 2019 06:19:42 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Byungchul Park <byungchul.park@lge.com>, 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
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 <20190708125013.GG26519@linux.ibm.com>
 <20190708130359.GA42888@google.com>
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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 <byungchul.park@lge.com>
> > > ---
> > >  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
> > > 
> > 
>