Date:   Sat, 13 Jul 2019 18:08:11 +0900
From:   Byungchul Park <max.byungchul.park@...il.com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>, josh@...htriplett.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <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 Fri, Jul 12, 2019 at 2:50 PM Byungchul Park <byungchul.park@....com> wrote:
>
> On Thu, Jul 11, 2019 at 05:30:52AM -0700, Paul E. McKenney wrote:
> > > > 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.  ;-)
> > >
> > > It makes sense, "A speculative tuning knobs do more harm than good".
> > >
> > > Then, it would be better to leave jiffies_till_{first,next}_fqs tunnable
> > > but jiffies_till_sched_qs until we need it.
> > >
> > > However,
> > >
> > > (1) In case that jiffies_till_sched_qs is tunnable:
> > >
> > >     We might need all of jiffies_till_{first,next}_qs,
> > >     jiffies_till_sched_qs and jiffies_to_sched_qs because
> > >     jiffies_to_sched_qs can be affected by any of them. So we
> > >     should be able to read each value at any time.
> > >
> > > (2) In case that jiffies_till_sched_qs is not tunnable:
> > >
> > >     I think we don't have to keep the jiffies_till_sched_qs any
> > >     longer since that's only for setting jiffies_to_sched_qs at
> > >     *booting time*, which can be done with jiffies_to_sched_qs too.
> > >     It's meaningless to keep all of tree variables.
> > >
> > > The simpler and less knobs that we really need we have, the better.
> > >
> > > what do you think about it?
> > >
> > > In the following patch, I (1) removed jiffies_till_sched_qs and then
> > > (2) renamed jiffies_*to*_sched_qs to jiffies_*till*_sched_qs because I
> > > think jiffies_till_sched_qs is a much better name for the purpose. I
> > > will resend it with a commit msg after knowing your opinion on it.
>
> Hi Paul,
>
> > I will give you a definite "maybe".
> >
> > Here are the two reasons for changing RCU's embarrassingly large array
> > of tuning parameters:
> >
> > 1.    They are causing a problem in production.  This would represent a
> >       bug that clearly must be fixed.  As you say, this change is not
> >       in this category.
> >
> > 2.    The change simplifies either RCU's code or the process of tuning
> >       RCU, but without degrading RCU's ability to run everywhere and
> >       without removing debugging tools.
>
> Agree.
>
> > The change below clearly simplifies things by removing a few lines of
> > code, and it does not change RCU's default self-configuration.  But are
> > we sure about the debugging aspect?  (Please keep in mind that many more
>
> I'm sorry I don't get it. I don't think this patch affect any debugging
> ability. What do you think it hurts? Could you explain it more?

I meant we are able to achieve it by directly changing
rcutree.jiffies_to_sched_qs even for the debugging purpose
w/o changing code itself as well. Thoughts?

-- 
Thanks,
Byungchul

> > sites are willing to change boot parameters than are willing to patch
> > their kernels.)
>
> Right.
>
> > What do you think?
> >
> > Finally, I urge you to join with Joel Fernandes and go through these
> > grace-period-duration tuning parameters.  Once you guys get your heads
> > completely around all of them and how they interact across the different
> > possible RCU configurations, I bet that the two of you will have excellent
> > ideas for improvement.
>
> Great. I'd be happy if I join the improvement and with Joel. I might
> need to ask you exactly what you expect in detail maybe. Anyway I will
> willingly go with it. :)
>
> Thanks,
> Byungchul
>
> >                                                       Thanx, Paul
> >
> > > 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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