Re: need_heavy_qs flag for PREEMPT=y kernels

["Paul E. McKenney" <paulmck@linux.ibm.com>]

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.

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 please see my earlier email on how things work out for kernels
built with NO_HZ_FULL=n&&PREEMPT=y.

							Thanx, Paul