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Date:   Mon, 21 May 2018 16:13:57 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Joel Fernandes <joelaf@google.com>
Cc:     linux-kernel@vger.kernel.org,
        Joel Fernandes <joel@joelfernandes.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>, byungchul.park@lge.com,
        kernel-team@android.com
Subject: Re: [PATCH v3 3/4] rcu: Use better variable names in funnel locking
 loop
Reply-To: paulmck@linux.vnet.ibm.com
References: <20180521044220.123933-1-joel@joelfernandes.org>
 <20180521044220.123933-4-joel@joelfernandes.org>
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On Sun, May 20, 2018 at 09:42:19PM -0700, Joel Fernandes wrote:
> The funnel locking loop in rcu_start_this_gp uses rcu_root as a
> temporary variable while walking the combining tree. This causes a
> tiresome exercise of a code reader reminding themselves that rcu_root
> may not be root. Lets just call it rnp, and rename other variables as
> well to be more appropriate.
> 
> Original patch: https://patchwork.kernel.org/patch/10396577/
> 
> Signed-off-by: Joel Fernandes <joel@joelfernandes.org>

Nice!

Please see feedback interspersed below.

						Thanx, Paul

> ---
>  kernel/rcu/tree.c | 48 ++++++++++++++++++++++++-----------------------
>  1 file changed, 25 insertions(+), 23 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 0ffd41ba304f..879c67a31116 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1526,7 +1526,7 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
> 
>  /*
>   * rcu_start_this_gp - Request the start of a particular grace period
> - * @rnp: The leaf node of the CPU from which to start.
> + * @rnp_start: The leaf node of the CPU from which to start.
>   * @rdp: The rcu_data corresponding to the CPU from which to start.
>   * @gp_seq_req: The gp_seq of the grace period to start.
>   *
> @@ -1540,12 +1540,12 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
>   *
>   * Returns true if the GP thread needs to be awakened else false.
>   */
> -static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
> +static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
>  			      unsigned long gp_seq_req)
>  {
>  	bool ret = false;
>  	struct rcu_state *rsp = rdp->rsp;
> -	struct rcu_node *rnp_root;
> +	struct rcu_node *rnp, *rnp_root = NULL;

Unless I am going blind, this patch really isn't using rnp_root.  It
could be removed.

> 
>  	/*
>  	 * Use funnel locking to either acquire the root rcu_node
> @@ -1556,34 +1556,36 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
>  	 * scan the leaf rcu_node structures.  Note that rnp->lock must
>  	 * not be released.
>  	 */
> -	raw_lockdep_assert_held_rcu_node(rnp);
> -	trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startleaf"));
> -	for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
> -		if (rnp_root != rnp)
> -			raw_spin_lock_rcu_node(rnp_root);
> -		if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_req) ||
> -		    rcu_seq_started(&rnp_root->gp_seq, gp_seq_req) ||
> -		    (rnp != rnp_root &&
> -		     rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
> -			trace_rcu_this_gp(rnp_root, rdp, gp_seq_req,
> +	raw_lockdep_assert_held_rcu_node(rnp_start);
> +	trace_rcu_this_gp(rnp_start, rdp, gp_seq_req, TPS("Startleaf"));
> +	for (rnp = rnp_start; 1; rnp = rnp->parent) {
> +		if (rnp != rnp_start)
> +			raw_spin_lock_rcu_node(rnp);
> +		if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req) ||
> +		    rcu_seq_started(&rnp->gp_seq, gp_seq_req) ||
> +		    (rnp != rnp_start &&
> +		     rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))) {
> +			trace_rcu_this_gp(rnp, rdp, gp_seq_req,
>  					  TPS("Prestarted"));
>  			goto unlock_out;
>  		}
> -		rnp_root->gp_seq_needed = gp_seq_req;
> -		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
> +		rnp->gp_seq_needed = gp_seq_req;
> +		if (rcu_seq_state(rcu_seq_current(&rnp_start->gp_seq))) {

The original had a performance bug, which is quite a bit more obvious
given the new names, so thank you for that!  The above statement should
instead be as follows:

		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {

It does not make sense to keep checking the starting rcu_node because
changes to ->gp_seq happen first at the top of the tree.  So we might
take an earlier exit by checking the current rnp instead of rechecking
rnp_start over and over.

Please feel free to make this change, which is probably best as a separate
patch.  That way this rename patch can remain a straightforward rename patch.

>  			/*
>  			 * We just marked the leaf, and a grace period
>  			 * is in progress, which means that rcu_gp_cleanup()
>  			 * will see the marking.  Bail to reduce contention.
>  			 */
> -			trace_rcu_this_gp(rnp, rdp, gp_seq_req,
> +			trace_rcu_this_gp(rnp_start, rdp, gp_seq_req,
>  					  TPS("Startedleaf"));
>  			goto unlock_out;
>  		}
> -		if (rnp_root != rnp && rnp_root->parent != NULL)
> -			raw_spin_unlock_rcu_node(rnp_root);
> -		if (!rnp_root->parent)
> +		if (rnp != rnp_start && rnp->parent != NULL)
> +			raw_spin_unlock_rcu_node(rnp);
> +		if (!rnp->parent) {
> +			rnp_root = rnp;

Since rnp_root is otherwise unused in the new version, the above statement
can be dropped along with the "if" statement's braces and the declaration.

>  			break;  /* At root, and perhaps also leaf. */
> +		}
>  	}
> 
>  	/* If GP already in progress, just leave, otherwise start one. */
> @@ -1601,11 +1603,11 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
>  	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq"));
>  	ret = true;  /* Caller must wake GP kthread. */
>  unlock_out:
> -	if (rnp != rnp_root)
> -		raw_spin_unlock_rcu_node(rnp_root);
> +	if (rnp != rnp_start)
> +		raw_spin_unlock_rcu_node(rnp);
>  	/* Push furthest requested GP to leaf node and rcu_data structure. */
> -	if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_req)) {
> -		rnp->gp_seq_needed = gp_seq_req;
> +	if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req)) {
> +		rnp_start->gp_seq_needed = gp_seq_req;
>  		rdp->gp_seq_needed = gp_seq_req;
>  	}
>  	return ret;
> -- 
> 2.17.0.441.gb46fe60e1d-goog
>