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Date:   Fri, 26 Jun 2020 13:44:48 -0700
From:   Jonathan Lemon <jonathan.lemon@gmail.com>
To:     =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@gmail.com>
Cc:     netdev@vger.kernel.org,
        =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@intel.com>,
        hch@lst.de, davem@davemloft.net, konrad.wilk@oracle.com,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        bpf@vger.kernel.org, maximmi@mellanox.com,
        magnus.karlsson@intel.com
Subject: Re: [PATCH net] xsk: remove cheap_dma optimization
Message-ID: <20200626204448.bxvr35qaxkfj6chs@bsd-mbp>
References: <20200626134358.90122-1-bjorn.topel@gmail.com>
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On Fri, Jun 26, 2020 at 03:43:58PM +0200, Bj?rn T?pel wrote:
> From: Bj?rn T?pel <bjorn.topel@intel.com>
> 
> When the AF_XDP buffer allocation API was introduced it had an
> optimization, "cheap_dma". The idea was that when the umem was DMA
> mapped, the pool also checked whether the mapping required a
> synchronization (CPU to device, and vice versa). If not, it would be
> marked as "cheap_dma" and the synchronization would be elided.
> 
> In [1] Christoph points out that the optimization above breaks the DMA
> API abstraction, and should be removed. Further, Christoph points out
> that optimizations like this should be done within the DMA mapping
> core, and not elsewhere.
> 
> Unfortunately this has implications for the packet rate
> performance. The AF_XDP rxdrop scenario shows a 9% decrease in packets
> per second.
> 
> [1] https://lore.kernel.org/netdev/20200626074725.GA21790@lst.de/
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Fixes: 2b43470add8c ("xsk: Introduce AF_XDP buffer allocation API")
> Signed-off-by: Bj?rn T?pel <bjorn.topel@intel.com>

Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>