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From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: rcu@vger.kernel.org, frederic@kernel.org
Subject: Re: need_heavy_qs flag for PREEMPT=y kernels
Date: Thu, 15 Aug 2019 13:31:07 -0700
Message-ID: <20190815203107.GL28441@linux.ibm.com> (raw)
In-Reply-To: <20190815200432.GB23194@google.com>

On Thu, Aug 15, 2019 at 04:04:32PM -0400, Joel Fernandes wrote:
> 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;
>  }

That one is not in my tree, either.  Frederic, should I add this to
your patch?  For that matter, may I add your Signed-off-by as well?
Your original is in my -rcu tree at:

0cb41806c799 ("EXP nohz: Add TICK_DEP_BIT_RCU")

I am testing Joel's suggested addition now.

							Thanx, Paul

> > 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]

  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
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 [this message]
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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