netdev - Re: [net V2 05/15] net/mlx5: Add retry mechanism to the command entry index allocation

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Message-ID: <20201001162336.45f552b3@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 1 Oct 2020 16:23:36 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     saeed@...nel.org
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Eran Ben Elisha <eranbe@...dia.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Moshe Shemesh <moshe@...dia.com>
Subject: Re: [net V2 05/15] net/mlx5: Add retry mechanism to the command
 entry index allocation

On Thu,  1 Oct 2020 12:52:37 -0700 saeed@...nel.org wrote:
> +static int cmd_alloc_index_retry(struct mlx5_cmd *cmd)
> +{
> +	unsigned long alloc_end = jiffies + msecs_to_jiffies(1000);
> +	int idx;
> +
> +retry:
> +	idx = cmd_alloc_index(cmd);
> +	if (idx < 0 && time_before(jiffies, alloc_end)) {
> +		/* Index allocation can fail on heavy load of commands. This is a temporary
> +		 * situation as the current command already holds the semaphore, meaning that
> +		 * another command completion is being handled and it is expected to release
> +		 * the entry index soon.
> +		 */
> +		cond_resched();
> +		goto retry;
> +	}
> +	return idx;
> +}

This looks excessive. At least add some cpu_relax(), or udelay()?