netdev - Re: [PATCH v2 net-next] mlx4: avoid unnecessary dirtying of critical fields

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Date:   Mon, 21 Nov 2016 11:33:44 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     ttoukan.linux@...il.com, netdev@...r.kernel.org,
        tariqt@...lanox.com
Subject: Re: [PATCH v2 net-next] mlx4: avoid unnecessary dirtying of
 critical fields

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sun, 20 Nov 2016 09:24:36 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> While stressing a 40Gbit mlx4 NIC with busy polling, I found false
> sharing in mlx4 driver that can be easily avoided.
> 
> This patch brings an additional 7 % performance improvement in UDP_RR
> workload.
> 
> 1) If we received no frame during one mlx4_en_process_rx_cq()
>    invocation, no need to call mlx4_cq_set_ci() and/or dirty ring->cons
> 
> 2) Do not refill rx buffers if we have plenty of them.
>    This avoids false sharing and allows some bulk/batch optimizations.
>    Page allocator and its locks will thank us.
> 
> Finally, mlx4_en_poll_rx_cq() should not return 0 if it determined
> cpu handling NIC IRQ should be changed. We should return budget-1
> instead, to not fool net_rx_action() and its netdev_budget.
> 
> 
> v2: keep AVG_PERF_COUNTER(... polled) even if polled is 0
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied.