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Date:   Tue, 9 Jul 2019 05:41:02 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Byungchul Park <byungchul.park@lge.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>, 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
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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 <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
> > > > 
> > > 
>