Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Jul 08, 2019 at 06:19:42AM -0700, Paul E. McKenney wrote:
[snip]
> > > 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.

Yes, agreed ;)

 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
> > > > 
> > > 
> > 
>