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From: Boris Pismenny <borispismenny@gmail.com>
To: David Ahern <dsahern@gmail.com>,
	Boris Pismenny <borisp@mellanox.com>,
	kuba@kernel.org, davem@davemloft.net, saeedm@nvidia.com,
	hch@lst.de, sagi@grimberg.me, axboe@fb.com, kbusch@kernel.org,
	viro@zeniv.linux.org.uk, edumazet@google.com
Cc: boris.pismenny@gmail.com, linux-nvme@lists.infradead.org,
	netdev@vger.kernel.org, benishay@nvidia.com, ogerlitz@nvidia.com,
	yorayz@nvidia.com, Ben Ben-Ishay <benishay@mellanox.com>,
	Or Gerlitz <ogerlitz@mellanox.com>,
	Yoray Zack <yorayz@mellanox.com>,
	borisp@nvidia.com
Subject: Re: [PATCH v1 net-next 01/15] iov_iter: Skip copy in memcpy_to_page if src==dst
Date: Tue, 8 Dec 2020 16:30:58 +0200	[thread overview]
Message-ID: <eb974de5-01c7-a4e1-b08d-d2e64bfed4a8@gmail.com> (raw)
In-Reply-To: <f399b37d-872b-c832-be43-c7930b487a16@gmail.com>


On 08/12/2020 2:39, David Ahern wrote:
> On 12/7/20 2:06 PM, Boris Pismenny wrote:
>> When using direct data placement the NIC writes some of the payload
>> directly to the destination buffer, and constructs the SKB such that it
>> points to this data. As a result, the skb_copy datagram_iter call will
>> attempt to copy data when it is not necessary.
>>
>> This patch adds a check to avoid this copy, and a static_key to enabled
>> it when TCP direct data placement is possible.
>>
> Why not mark the skb as ZEROCOPY -- an Rx version of the existing
> SKBTX_DEV_ZEROCOPY and skb_shared_info->tx_flags? Use that as a generic
> way of indicating to the stack what is happening.
>
>

[Re-sending as the previous one didn't hit the mailing list]

Interesting idea. But, unlike SKBTX_DEV_ZEROCOPY this SKB can be inspected/modified by the stack without the need to copy things out. Additionally, the SKB may contain both data that is already placed in its final destination buffer (PDU data) and data that isn't (PDU header); it doesn't matter. Therefore, labeling the entire SKB as zerocopy doesn't convey the desired information. Moreover, skipping copies in the stack to receive zerocopy SKBs will require more invasive changes.

Our goal in this approach was to provide the smallest change that enables the desired functionality while preserving the performance of existing flows that do not care for it. An alternative approach, that doesn't affect existing flows at all, which we considered was to make a special version of memcpy_to_page to be used by DDP providers (nvme-tcp). This alternative will require creating corresponding special versions for users of this function such skb_copy_datagram_iter. Thit is more invasive, thus in this patchset we decided to avoid it.

  reply	other threads:[~2020-12-08 14:31 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-07 21:06 [PATCH v1 net-next 00/15] nvme-tcp receive offloads Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 01/15] iov_iter: Skip copy in memcpy_to_page if src==dst Boris Pismenny
2020-12-08  0:39   ` David Ahern
2020-12-08 14:30     ` Boris Pismenny [this message]
2020-12-07 21:06 ` [PATCH v1 net-next 02/15] net: Introduce direct data placement tcp offload Boris Pismenny
2020-12-08  0:42   ` David Ahern
2020-12-08 14:36     ` Boris Pismenny
2020-12-09  0:38       ` David Ahern
2020-12-09  8:15         ` Boris Pismenny
2020-12-10  4:26           ` David Ahern
2020-12-11  2:01             ` Jakub Kicinski
2020-12-11  2:43               ` David Ahern
2020-12-11 18:45                 ` Jakub Kicinski
2020-12-11 18:58                   ` Eric Dumazet
2020-12-11 19:59                   ` David Ahern
2020-12-11 23:05                     ` Jonathan Lemon
2020-12-13 18:34                   ` Boris Pismenny
2020-12-13 18:21             ` Boris Pismenny
2020-12-15  5:19               ` David Ahern
2020-12-17 19:06                 ` Boris Pismenny
2020-12-18  0:44                   ` David Ahern
2020-12-09  0:57   ` David Ahern
2020-12-09  1:11     ` David Ahern
2020-12-09  8:28       ` Boris Pismenny
2020-12-09  8:25     ` Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 03/15] net: Introduce crc offload for tcp ddp ulp Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 04/15] net/tls: expose get_netdev_for_sock Boris Pismenny
2020-12-09  1:06   ` David Ahern
2020-12-09  7:41     ` Boris Pismenny
2020-12-10  3:39       ` David Ahern
2020-12-11 18:43         ` Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 05/15] nvme-tcp: Add DDP offload control path Boris Pismenny
2020-12-10 17:15   ` Shai Malin
2020-12-14  6:38     ` Boris Pismenny
2020-12-15 13:33       ` Shai Malin
2020-12-17 18:51         ` Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 06/15] nvme-tcp: Add DDP data-path Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 07/15] nvme-tcp : Recalculate crc in the end of the capsule Boris Pismenny
2020-12-15 14:07   ` Shai Malin
2020-12-07 21:06 ` [PATCH v1 net-next 08/15] nvme-tcp: Deal with netdevice DOWN events Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 09/15] net/mlx5: Header file changes for nvme-tcp offload Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 10/15] net/mlx5: Add 128B CQE for NVMEoTCP offload Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 11/15] net/mlx5e: TCP flow steering for nvme-tcp Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 12/15] net/mlx5e: NVMEoTCP DDP offload control path Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 13/15] net/mlx5e: NVMEoTCP, data-path for DDP offload Boris Pismenny
2020-12-18  0:57   ` David Ahern
2020-12-07 21:06 ` [PATCH v1 net-next 14/15] net/mlx5e: NVMEoTCP statistics Boris Pismenny
2020-12-07 21:06 ` [PATCH v1 net-next 15/15] net/mlx5e: NVMEoTCP workaround CRC after resync Boris Pismenny
2021-01-14  1:27 ` [PATCH v1 net-next 00/15] nvme-tcp receive offloads Sagi Grimberg
2021-01-14  4:47   ` David Ahern
2021-01-14 19:21     ` Boris Pismenny
2021-01-14 19:17   ` Boris Pismenny
2021-01-14 21:07     ` Sagi Grimberg

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