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From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: rcu <rcu@vger.kernel.org>
Subject: Re: need_heavy_qs flag for PREEMPT=y kernels
Date: Sun, 11 Aug 2019 16:30:24 -0700
Message-ID: <20190811233024.GZ28441@linux.ibm.com> (raw)
In-Reply-To: <20190811212505.GB128944@google.com>

On Sun, Aug 11, 2019 at 05:25:05PM -0400, Joel Fernandes wrote:
> On Sun, Aug 11, 2019 at 02:16:46PM -0700, Paul E. McKenney wrote:
> > On Sun, Aug 11, 2019 at 02:34:08PM -0400, Joel Fernandes wrote:
> > > On Sun, Aug 11, 2019 at 2:08 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > > >
> > > > Hi Paul, everyone,
> > > >
> > > > I noticed on reading code that the need_heavy_qs check and
> > > > rcu_momentary_dyntick_idle() is only called for !PREEMPT kernels. Don't we
> > > > need to call this for PREEMPT kernels for the benefit of nohz_full CPUs?
> > > >
> > > > Consider the following events:
> > > > 1. Kernel is PREEMPT=y configuration.
> > > > 2. CPU 2 is a nohz_full CPU running only a single task and the tick is off.
> > > > 3. CPU 2 is running only in kernel mode and does not enter user mode or idle.
> > > > 4. Grace period thread running on CPU 3 enter the fqs loop.
> > > > 5. Enough time passes and it sets the need_heavy_qs for CPU2.
> > > > 6. CPU 2 is still in kernel mode but does cond_resched().
> > > > 7. cond_resched() does not call rcu_momentary_dyntick_idle() because PREEMPT=y.
> > > >
> > > > Is 7. not calling rcu_momentary_dyntick_idle() a lost opportunity for the FQS
> > > > loop to detect that the CPU has crossed a quiescent point?
> > > >
> > > > Is this done so that cond_resched() is fast for PREEMPT=y kernels?
> > > 
> > > Oh, so I take it this bit of code in rcu_implicit_dynticks_qs(), with
> > > the accompanying comments, takes care of the scenario I describe?
> > > Another way could be just call rcu_momentary_dyntick_idle() during
> > > cond_resched() for nohz_full CPUs? Is that pricey?
> > >         /*
> > >          * NO_HZ_FULL CPUs can run in-kernel without rcu_sched_clock_irq!
> > >          * The above code handles this, but only for straight cond_resched().
> > >          * And some in-kernel loops check need_resched() before calling
> > >          * cond_resched(), which defeats the above code for CPUs that are
> > >          * running in-kernel with scheduling-clock interrupts disabled.
> > >          * So hit them over the head with the resched_cpu() hammer!
> > >          */
> > >         if (tick_nohz_full_cpu(rdp->cpu) &&
> > >                    time_after(jiffies,
> > >                               READ_ONCE(rdp->last_fqs_resched) + jtsq * 3)) {
> > >                 resched_cpu(rdp->cpu);
> > >                 WRITE_ONCE(rdp->last_fqs_resched, jiffies);
> > >         }
> > 
> > Yes, for NO_HZ_FULL=y&&PREEMPT=y kernels.
> 
> Actually, I was only referring to the case of NO_HZ_FULL=y being the
> troublesome one (i.e. rcu_need_heavy_qs flag would have no effect).
> 
> For NO_HZ_FULL=n, I have full confidence the scheduler tick will notice
> rcu_urgent_qs and do a reschedule. The ensuing softirq then does the needful
> to help end the grace period.

Whew!

That confidence was not at all apparent in your initial email.

> > Your thought of including rcu_momentary_dyntick_idle() would function
> > correctly, but would cause performance issues.  Even adding additional
> > compares and branches in that hot codepath is visible to 0day test robot!
> > So adding a read-modify-write atomic operation to that code path would
> > get attention of the wrong kind.  ;-)
> 
> But wouldn't these performance issues also be visible with
> NO_HZ_FULL=y && PREEMPT=n?

In PREEMPT=n, cond_resched() already has a check, and with quite a bit
of care it is possible to introduce another.

>                             Why is PREEMPT=n made an exception?

The exception is actually CONFIG_NO_HZ_FULL=y && CONFIG_PREEMPT=y.
In that case, we can rely on neither the scheduling-clock interrupt
nor on cond_resched().  In the other three cases, we have one or both.

>                                                                 Is it that
> 0day doesn't test this combination much? :-D

Might be, but it sure tests the other combinations!

Next question:  Why does rcu_implicit_dynticks_qs() check only for
tick_nohz_full_cpu() and not also IS_ENABLED(CONFIG_PREEMPT)?  After
all, a nohz_full CPU in a !CONFIG_PREEMPT kernel should be able to
rely on cond_resched(), right?

Should this change?  Why or why not?

							Thanx, Paul

  reply index

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-11 18:08 Joel Fernandes
2019-08-11 18:34 ` Joel Fernandes
2019-08-11 21:16   ` Paul E. McKenney
2019-08-11 21:25     ` Joel Fernandes
2019-08-11 23:30       ` Paul E. McKenney [this message]
2019-08-12  1:24         ` Joel Fernandes
2019-08-12  1:40           ` Joel Fernandes
2019-08-12  3:57             ` Paul E. McKenney
2019-08-11 21:13 ` Paul E. McKenney
2019-08-12  3:21   ` Joel Fernandes
2019-08-12  3:53     ` Paul E. McKenney
2019-08-12 21:20       ` Joel Fernandes
2019-08-12 23:01         ` Paul E. McKenney
2019-08-13  1:02           ` Joel Fernandes
2019-08-13  1:05             ` Joel Fernandes
2019-08-13  2:28               ` Paul E. McKenney
2019-08-13  2:27             ` Paul E. McKenney
2019-08-13  2:50               ` Paul E. McKenney
2019-08-15 17:17             ` Paul E. McKenney
2019-08-15 20:04               ` Joel Fernandes
2019-08-15 20:31                 ` Paul E. McKenney
2019-08-15 21:22                   ` Joel Fernandes
2019-08-15 21:27                     ` Joel Fernandes
2019-08-15 21:34                       ` Joel Fernandes
2019-08-15 21:57                         ` Paul E. McKenney
2019-08-15 21:45                     ` Paul E. McKenney
2019-08-16  0:02                       ` Joel Fernandes
2019-08-19 12:34                         ` Frederic Weisbecker
2019-08-19 12:09                   ` Frederic Weisbecker
2019-08-19 16:57                   ` Frederic Weisbecker
2019-08-19 22:31                     ` Paul E. McKenney

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