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From: Joel Fernandes <joel@joelfernandes.org>
To: linux-kernel@vger.kernel.org, "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: rcu@vger.kernel.org, frederic@kernel.org
Subject: Re: [PATCH -rcu dev 3/3] RFC: rcu/tree: Read dynticks_nmi_nesting in advance
Date: Fri, 16 Aug 2019 12:24:04 -0400
Message-ID: <20190816162404.GB10481@google.com> (raw)
In-Reply-To: <20190816025311.241257-3-joel@joelfernandes.org>

On Thu, Aug 15, 2019 at 10:53:11PM -0400, Joel Fernandes (Google) wrote:
> I really cannot explain this patch, but without it, the "else if" block
> just doesn't execute thus causing the tick's dep mask to not be set and
> causes the tick to be turned off.
> 
> I tried various _ONCE() macros but the only thing that works is this
> patch.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/rcu/tree.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 856d3c9f1955..ac6bcf7614d7 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -802,6 +802,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
>  {
>  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  	long incby = 2;
> +	int dnn = rdp->dynticks_nmi_nesting;

I believe the accidental sign extension / conversion from long to int was
giving me an illusion since things started working well. Changing the 'int
dnn' to 'long dnn' gives similar behavior as without this patch! At least I
know now. Please feel free to ignore this particular RFC patch while I debug
this more (over the weekend or early next week). The first 2 patches are
good, just ignore this one.

thanks,

 - Joel


>  
>  	/* Complain about underflow. */
>  	WARN_ON_ONCE(rdp->dynticks_nmi_nesting < 0);
> @@ -826,7 +827,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
>  
>  		incby = 1;
>  	} else if (tick_nohz_full_cpu(rdp->cpu) &&
> -		   !rdp->dynticks_nmi_nesting &&
> +		   !dnn &&
>  		   rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
>  		rdp->rcu_forced_tick = true;
>  		tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
> -- 
> 2.23.0.rc1.153.gdeed80330f-goog
> 

  reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-16  2:53 [PATCH -rcu dev 1/3] rcu/tree: tick_dep_set/clear_cpu should accept bits instead of masks Joel Fernandes (Google)
2019-08-16  2:53 ` [PATCH -rcu dev 2/3] rcu/tree: Fix issue where sometimes rcu_urgent_qs is not set on IPI Joel Fernandes (Google)
2019-08-16  2:57   ` Joel Fernandes
2019-08-16  2:53 ` [PATCH -rcu dev 3/3] RFC: rcu/tree: Read dynticks_nmi_nesting in advance Joel Fernandes (Google)
2019-08-16 16:24   ` Joel Fernandes [this message]
2019-08-16 16:52     ` Paul E. McKenney
2019-08-16 17:07       ` Joel Fernandes
2019-08-16 17:30         ` Paul E. McKenney
2019-08-19 12:59       ` Frederic Weisbecker
2019-08-19 14:22         ` Joel Fernandes
2019-08-19 14:41           ` Paul E. McKenney
2019-08-19 15:46             ` Joel Fernandes
2019-08-19 16:17               ` Paul E. McKenney
2019-08-19 14:40         ` Paul E. McKenney
2019-08-16  2:59 ` [PATCH v2 -rcu dev 2/3] rcu/tree: Fix issue where sometimes rcu_urgent_qs is not set on IPI Joel Fernandes (Google)
2019-08-16 17:04   ` Paul E. McKenney
2019-08-16 17:25 ` [PATCH -rcu dev 1/3] rcu/tree: tick_dep_set/clear_cpu should accept bits instead of masks Paul E. McKenney
2019-08-16 17:35   ` Joel Fernandes
2019-08-19 12:38 ` Frederic Weisbecker
2019-08-19 14:46   ` Paul E. McKenney
2019-08-19 16:32     ` Frederic Weisbecker
2019-08-19 16:44       ` Paul E. McKenney
2019-08-20 12:08         ` Frederic Weisbecker
2019-08-20 14:44           ` Paul E. McKenney

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