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Message-Id: <20190708131942.GH26519@linux.ibm.com>
Date: Mon, 8 Jul 2019 06:19:42 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Byungchul Park <byungchul.park@....com>, josh@...htriplett.org,
rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
jiangshanlai@...il.com, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH] rcu: Make jiffies_till_sched_qs writable
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@....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
> > >
> >
>
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