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SubjectRe: [PATCH] rcu: Make jiffies_till_sched_qs writable
On Thu, Jul 11, 2019 at 08:02:15AM -0700, Paul E. McKenney wrote:
> These would be the tunables controlling how quickly RCU takes its
> various actions to encourage the current grace period to end quickly.

Seriously one of the most interesting thing over all kernel works.

> I would be happy to give you the exact list if you wish, but most of
> them have appeared in this thread.

Thank you. :)

> The experiments should be designed to work out whether the current
> default settings have configurations where they act badly. This might
> also come up with advice for people attempting hand-tuning, or proposed
> parameter-checking code to avoid bad combinations.

Great.

> For one example, setting the RCU CPU stall timeout too low will definitely
> cause some unwanted splats. (Yes, one could argue that other things in
> the kernel should change to allow this value to decrease, but things
> like latency tracer and friends are probably more useful and important.)

Agree.

Thanks,
Byungchul

>
> Thanx, Paul
>
> > thanks,
> >
> > - Joel
> >
> >
> >
> > > > Thanks,
> > > > Byungchul
> > > >
> > > > ---8<---
> > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > > index e72c184..94b58f5 100644
> > > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > > @@ -3792,10 +3792,6 @@
> > > > a value based on the most recent settings
> > > > of rcutree.jiffies_till_first_fqs
> > > > and rcutree.jiffies_till_next_fqs.
> > > > - This calculated value may be viewed in
> > > > - rcutree.jiffies_to_sched_qs. Any attempt to set
> > > > - rcutree.jiffies_to_sched_qs will be cheerfully
> > > > - overwritten.
> > > >
> > > > rcutree.kthread_prio= [KNL,BOOT]
> > > > Set the SCHED_FIFO priority of the RCU per-CPU
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index a2f8ba2..ad9dc86 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -421,10 +421,8 @@ static int rcu_is_cpu_rrupt_from_idle(void)
> > > > * How long the grace period must be before we start recruiting
> > > > * quiescent-state help from rcu_note_context_switch().
> > > > */
> > > > -static ulong jiffies_till_sched_qs = ULONG_MAX;
> > > > +static ulong jiffies_till_sched_qs = ULONG_MAX; /* See adjust_jiffies_till_sched_qs(). */
> > > > 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
> > > > @@ -436,18 +434,13 @@ static void adjust_jiffies_till_sched_qs(void)
> > > > {
> > > > unsigned long j;
> > > >
> > > > - /* If jiffies_till_sched_qs was specified, respect the request. */
> > > > - if (jiffies_till_sched_qs != ULONG_MAX) {
> > > > - WRITE_ONCE(jiffies_to_sched_qs, jiffies_till_sched_qs);
> > > > - return;
> > > > - }
> > > > /* Otherwise, set to third fqs scan, but bound below on large system. */
> > > > j = READ_ONCE(jiffies_till_first_fqs) +
> > > > 2 * READ_ONCE(jiffies_till_next_fqs);
> > > > if (j < HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV)
> > > > j = HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
> > > > pr_info("RCU calculated value of scheduler-enlistment delay is %ld jiffies.\n", j);
> > > > - WRITE_ONCE(jiffies_to_sched_qs, j);
> > > > + WRITE_ONCE(jiffies_till_sched_qs, j);
> > > > }
> > > >
> > > > static int param_set_first_fqs_jiffies(const char *val, const struct kernel_param *kp)
> > > > @@ -1033,16 +1026,16 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> > > >
> > > > /*
> > > > * A CPU running for an extended time within the kernel can
> > > > - * delay RCU grace periods: (1) At age jiffies_to_sched_qs,
> > > > - * set .rcu_urgent_qs, (2) At age 2*jiffies_to_sched_qs, set
> > > > + * delay RCU grace periods: (1) At age jiffies_till_sched_qs,
> > > > + * set .rcu_urgent_qs, (2) At age 2*jiffies_till_sched_qs, set
> > > > * both .rcu_need_heavy_qs and .rcu_urgent_qs. Note that the
> > > > * unsynchronized assignments to the per-CPU rcu_need_heavy_qs
> > > > * variable are safe because the assignments are repeated if this
> > > > * CPU failed to pass through a quiescent state. This code
> > > > - * also checks .jiffies_resched in case jiffies_to_sched_qs
> > > > + * also checks .jiffies_resched in case jiffies_till_sched_qs
> > > > * is set way high.
> > > > */
> > > > - jtsq = READ_ONCE(jiffies_to_sched_qs);
> > > > + jtsq = READ_ONCE(jiffies_till_sched_qs);
> > > > ruqp = per_cpu_ptr(&rcu_data.rcu_urgent_qs, rdp->cpu);
> > > > rnhqp = &per_cpu(rcu_data.rcu_need_heavy_qs, rdp->cpu);
> > > > if (!READ_ONCE(*rnhqp) &&
> > > > @@ -3383,7 +3376,8 @@ static void __init rcu_init_geometry(void)
> > > > jiffies_till_first_fqs = d;
> > > > if (jiffies_till_next_fqs == ULONG_MAX)
> > > > jiffies_till_next_fqs = d;
> > > > - adjust_jiffies_till_sched_qs();
> > > > + if (jiffies_till_sched_qs == ULONG_MAX)
> > > > + adjust_jiffies_till_sched_qs();
> > > >
> > > > /* If the compile-time values are accurate, just leave. */
> > > > if (rcu_fanout_leaf == RCU_FANOUT_LEAF &&
> > >

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