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

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Date:   Wed, 6 Feb 2019 17:40:06 +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] mlx5: use RCU lock in mlx5_eq_cq_get()

On Wed, 2019-02-06 at 09:35 -0800, Saeed Mahameed wrote:
> On Tue, 2019-02-05 at 16:35 -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. It 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% with our
> > workload.
> > 
> > Cc: Saeed Mahameed <saeedm@...lanox.com>
> > Cc: Tariq Toukan <tariqt@...lanox.com>
> > Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> 
> Acked-by: Saeed Mahameed <saeedm@...lanox.com>
> 

Actually, the commit message needs some rework, since there is no
contention upstream, Cong can you take care of this and post a V2 ?

Thanks,
Saeed.