netdev - Re: [PATCH net-next 4/8] net/mlx5e: Consider IRQ affinity changes in NAPI poll

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Message-Id: <20151101.173407.1794026161201499991.davem@davemloft.net>
Date:	Sun, 01 Nov 2015 17:34:07 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	ogerlitz@...lanox.com
Cc:	netdev@...r.kernel.org, saeedm@...lanox.com, achiad@...lanox.com
Subject: Re: [PATCH net-next 4/8] net/mlx5e: Consider IRQ affinity changes
 in NAPI poll

From: Or Gerlitz <ogerlitz@...lanox.com>
Date: Sun,  1 Nov 2015 19:35:18 +0200

> @@ -49,6 +50,15 @@ struct mlx5_cqe64 *mlx5e_get_cqe(struct mlx5e_cq *cq)
>  	return cqe;
>  }
>  
> +static inline bool mlx5e_no_channel_affinity_change(struct mlx5e_channel *c)
> +{
> +	int current_cpu = smp_processor_id();
> +	struct irq_data *d = irq_desc_get_irq_data(c->irq_desc);
> +	struct cpumask *aff = irq_data_get_affinity_mask(d);
> +
> +	return cpumask_test_cpu(current_cpu, aff);
> +}

This is so much pointer dereferencing and then a bitmask test as well.

Are you really sure sure an extremely rare situation warrants this
test every single NAPI poll call?

If this is a real problem, then every driver is susceptible to the
issue and it therefore warrants a generic solution.  And if we have
generic infrastructure for this situation in the code NAPI polling
networking code, I guarantee that it will probably be implemented much
more cheaply than this.
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