From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: rcu <rcu@vger.kernel.org>
Subject: Re: need_heavy_qs flag for PREEMPT=y kernels
Date: Sun, 11 Aug 2019 17:25:05 -0400 [thread overview]
Message-ID: <20190811212505.GB128944@google.com> (raw)
In-Reply-To: <20190811211646.GY28441@linux.ibm.com>
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.
> 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? Why is PREEMPT=n made an exception? Is it that
0day doesn't test this combination much? :-D
thanks,
- Joel
next prev parent reply other threads:[~2019-08-11 21:25 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-11 18:08 need_heavy_qs flag for PREEMPT=y kernels Joel Fernandes
2019-08-11 18:34 ` Joel Fernandes
2019-08-11 21:16 ` Paul E. McKenney
2019-08-11 21:25 ` Joel Fernandes [this message]
2019-08-11 23:30 ` Paul E. McKenney
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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