From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: rcu@vger.kernel.org
Subject: Re: need_heavy_qs flag for PREEMPT=y kernels
Date: Thu, 15 Aug 2019 16:04:32 -0400 [thread overview]
Message-ID: <20190815200432.GB23194@google.com> (raw)
In-Reply-To: <20190815171714.GA1023@linux.ibm.com>
On Thu, Aug 15, 2019 at 10:17:14AM -0700, Paul E. McKenney wrote:
> On Mon, Aug 12, 2019 at 09:02:49PM -0400, Joel Fernandes wrote:
> > On Mon, Aug 12, 2019 at 04:01:38PM -0700, Paul E. McKenney wrote:
>
> [ . . . ]
>
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 8c494a692728..ad906d6a74fb 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -651,6 +651,12 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
> > > */
> > > if (rdp->dynticks_nmi_nesting != 1) {
> > > trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, rdp->dynticks);
> > > + if (tick_nohz_full_cpu(rdp->cpu) &&
> > > + rdp->dynticks_nmi_nesting == 2 &&
> > > + rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
> > > + rdp->rcu_forced_tick = true;
> > > + tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU);
> > > + }
> >
> >
> > Instead of checking dynticks_nmi_nesting == 2 in rcu_nmi_exit_common(), can
> > we do the tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU) from
> > rcu_nmi_enter_common() ? We could add this code there, under the "if
> > (rcu_dynticks_curr_cpu_in_eqs())".
>
> This would need to go in an "else" clause, correct? But there would still
> want to be a check for interrupt from base level (which would admittedly
> be an equality comparison with zero) and we would also still need to check
> for rdp->rcu_urgent_qs && !rdp->rcu_forced_tick.
True, agreed. I replied to this before saying it should be
!rcu_dynticks_curr_cpu_in_eqs() in the "if" ;) But it seems I could also be
missing the check for TICK_DEP_MASK_RCU in my tree so I think we need this as
well which is below as diff. Testing it more now!
And, with this I do get many more ticks during the test. But there are
intervals where the tick is not seen. Still it is much better than before:
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index be9707f68024..e697c7a2ce67 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -198,6 +198,10 @@ static bool check_tick_dependency(atomic_t *dep)
return true;
}
+ if (val & TICK_DEP_MASK_CLOCK_RCU) {
+ return true;
+ }
+
return false;
}
> Still, an equal-zero comparison is probably going to be a bit cheaper than
> an equals-two comparison, and this is on the interrupt-entry fastpath,
> so this change is likely worth making. Good call!!!
Thanks!!
- Joel
[snip]
next prev parent reply other threads:[~2019-08-15 20:04 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
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 [this message]
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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