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.
> 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 :)
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@xxxxxxx>
> > ---
> > 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
> >
>
