netdev - Re: [Patch net-next v2] mlx5: use RCU lock in mlx5_eq_cq

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Date:   Mon, 11 Feb 2019 22:41:51 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>
CC:     Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [Patch net-next v2] mlx5: use RCU lock in mlx5_eq_cq_get()

On Wed, 2019-02-06 at 15:00 -0800, Cong Wang wrote:
> mlx5_eq_cq_get() is called in IRQ handler, the spinlock inside
> gets a lot of contentions when we test some heavy workload
> with 60 RX queues and 80 CPU's, and it is clearly shown in the
> flame graph.
> 
> In fact, radix_tree_lookup() is perfectly fine with RCU read lock,
> we don't have to take a spinlock on this hot path. This is pretty
> much similar to commit 291c566a2891
> ("net/mlx4_core: Fix racy CQ (Completion Queue) free"). Slow paths
> are still serialized with the spinlock, and with synchronize_irq()
> it should be safe to just move the fast path to RCU read lock.
> 
> This patch itself reduces the latency by about 50% for our memcached
> workload on a 4.14 kernel we test. In upstream, as pointed out by
> Saeed,
> this spinlock gets some rework in commit 02d92f790364
> ("net/mlx5: CQ Database per EQ"), so the difference could be smaller.
> 
> Cc: Saeed Mahameed <saeedm@...lanox.com>
> Cc: Tariq Toukan <tariqt@...lanox.com>
> Acked-by: Saeed Mahameed <saeedm@...lanox.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> 

Applied to mlx5-next

Will be sent to net-next in my next pull request

Thanks!