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@xxxxxxxxxxxxxxxxx> 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
