linux-kernel - Re: [PATCH net-next v2 7/8] net/mlx5: HWS, Shrink empty matchers

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Message-ID: <20250624170809.2aac2c69@kernel.org>
Date: Tue, 24 Jun 2025 17:08:09 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Mark Bloch <mbloch@...dia.com>
Cc: "David S. Miller" <davem@...emloft.net>, Paolo Abeni
 <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>, "Andrew Lunn"
 <andrew+netdev@...n.ch>, Simon Horman <horms@...nel.org>,
 <saeedm@...dia.com>, <gal@...dia.com>, <leonro@...dia.com>,
 <tariqt@...dia.com>, Leon Romanovsky <leon@...nel.org>,
 <netdev@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <moshe@...dia.com>, Yevgeny Kliteynik
 <kliteyn@...dia.com>, Vlad Dogaru <vdogaru@...dia.com>
Subject: Re: [PATCH net-next v2 7/8] net/mlx5: HWS, Shrink empty matchers

On Sun, 22 Jun 2025 20:22:25 +0300 Mark Bloch wrote:
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/steering/hws/bwc.c b/drivers/net/ethernet/mellanox/mlx5/core/steering/hws/bwc.c
> index 0a7903cf75e8..b7098c7d2112 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/steering/hws/bwc.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/steering/hws/bwc.c
> @@ -3,6 +3,8 @@
>  
>  #include "internal.h"
>  
> +static int hws_bwc_matcher_move(struct mlx5hws_bwc_matcher *bwc_matcher);

Is there a circular dependency? Normally we recommend that people
reorder code rather that add forward declarations.

>  static u16 hws_bwc_gen_queue_idx(struct mlx5hws_context *ctx)
>  {
>  	/* assign random queue */
> @@ -409,6 +411,70 @@ static void hws_bwc_rule_cnt_dec(struct mlx5hws_bwc_rule *bwc_rule)
>  		atomic_dec(&bwc_matcher->tx_size.num_of_rules);
>  }
>  
> +static int
> +hws_bwc_matcher_rehash_shrink(struct mlx5hws_bwc_matcher *bwc_matcher)
> +{
> +	struct mlx5hws_bwc_matcher_size *rx_size = &bwc_matcher->rx_size;
> +	struct mlx5hws_bwc_matcher_size *tx_size = &bwc_matcher->tx_size;
> +
> +	/* It is possible that another thread has added a rule.
> +	 * Need to check again if we really need rehash/shrink.
> +	 */
> +	if (atomic_read(&rx_size->num_of_rules) ||
> +	    atomic_read(&tx_size->num_of_rules))
> +		return 0;
> +
> +	/* If the current matcher RX/TX size is already at its initial size. */
> +	if (rx_size->size_log == MLX5HWS_BWC_MATCHER_INIT_SIZE_LOG &&
> +	    tx_size->size_log == MLX5HWS_BWC_MATCHER_INIT_SIZE_LOG)
> +		return 0;
> +
> +	/* Now we've done all the checking - do the shrinking:
> +	 *  - reset match RTC size to the initial size
> +	 *  - create new matcher
> +	 *  - move the rules, which will not do anything as the matcher is empty
> +	 *  - destroy the old matcher
> +	 */
> +
> +	rx_size->size_log = MLX5HWS_BWC_MATCHER_INIT_SIZE_LOG;
> +	tx_size->size_log = MLX5HWS_BWC_MATCHER_INIT_SIZE_LOG;
> +
> +	return hws_bwc_matcher_move(bwc_matcher);
> +}
> +
> +static int hws_bwc_rule_cnt_dec_with_shrink(struct mlx5hws_bwc_rule *bwc_rule,
> +					    u16 bwc_queue_idx)
> +{
> +	struct mlx5hws_bwc_matcher *bwc_matcher = bwc_rule->bwc_matcher;
> +	struct mlx5hws_context *ctx = bwc_matcher->matcher->tbl->ctx;
> +	struct mutex *queue_lock; /* Protect the queue */
> +	int ret;
> +
> +	hws_bwc_rule_cnt_dec(bwc_rule);
> +
> +	if (atomic_read(&bwc_matcher->rx_size.num_of_rules) ||
> +	    atomic_read(&bwc_matcher->tx_size.num_of_rules))
> +		return 0;
> +
> +	/* Matcher has no more rules - shrink it to save ICM. */
> +
> +	queue_lock = hws_bwc_get_queue_lock(ctx, bwc_queue_idx);
> +	mutex_unlock(queue_lock);
> +
> +	hws_bwc_lock_all_queues(ctx);
> +	ret = hws_bwc_matcher_rehash_shrink(bwc_matcher);
> +	hws_bwc_unlock_all_queues(ctx);
> +
> +	mutex_lock(queue_lock);

Dropping and re-taking caller-held locks is a bad code smell.
Please refactor - presumably you want some portion of the condition
to be under the lock with the dec? return true / false based on that.
let the caller drop the lock and do the shrink if true was returned
(directly or with another helper)

> +	if (unlikely(ret))
> +		mlx5hws_err(ctx,
> +			    "BWC rule deletion: shrinking empty matcher failed (%d)\n",
> +			    ret);
> +
> +	return ret;
> +}
> +
>  int mlx5hws_bwc_rule_destroy_simple(struct mlx5hws_bwc_rule *bwc_rule)
>  {
>  	struct mlx5hws_bwc_matcher *bwc_matcher = bwc_rule->bwc_matcher;
> @@ -425,8 +491,8 @@ int mlx5hws_bwc_rule_destroy_simple(struct mlx5hws_bwc_rule *bwc_rule)
>  	mutex_lock(queue_lock);
>  
>  	ret = hws_bwc_rule_destroy_hws_sync(bwc_rule, &attr);
> -	hws_bwc_rule_cnt_dec(bwc_rule);
>  	hws_bwc_rule_list_remove(bwc_rule);
> +	hws_bwc_rule_cnt_dec_with_shrink(bwc_rule, idx);
>  
>  	mutex_unlock(queue_lock);