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From: Boris Pismenny <borispismenny@gmail.com>
To: Eric Dumazet <edumazet@google.com>
Cc: Boris Pismenny <borisp@nvidia.com>,
	David Ahern <dsahern@gmail.com>, Jakub Kicinski <kuba@kernel.org>,
	David Miller <davem@davemloft.net>,
	Saeed Mahameed <saeedm@nvidia.com>,
	Christoph Hellwig <hch@lst.de>,
	sagi@grimberg.me, axboe@fb.com, kbusch@kernel.org,
	Al Viro <viro@zeniv.linux.org.uk>,
	smalin@marvell.com, boris.pismenny@gmail.com,
	linux-nvme@lists.infradead.org, netdev <netdev@vger.kernel.org>,
	benishay@nvidia.com, ogerlitz@nvidia.com, yorayz@nvidia.com,
	Boris Pismenny <borisp@mellanox.com>,
	Ben Ben-Ishay <benishay@mellanox.com>,
	Or Gerlitz <ogerlitz@mellanox.com>,
	Yoray Zack <yorayz@mellanox.com>
Subject: Re: [PATCH v5 net-next 01/36] net: Introduce direct data placement tcp offload
Date: Thu, 22 Jul 2021 16:33:41 +0300	[thread overview]
Message-ID: <ba72f780-840e-de73-31b3-137908c52868@gmail.com> (raw)
In-Reply-To: <CANn89iLUDcL-F2RvaNz5+b8oQPnL1DnanHe0vvMb8QkM26whCQ@mail.gmail.com>

On 22/07/2021 16:10, Eric Dumazet wrote:
> On Thu, Jul 22, 2021 at 2:18 PM Boris Pismenny <borispismenny@gmail.com> wrote:
>>
>> On 22/07/2021 14:26, Eric Dumazet wrote:
>>> On Thu, Jul 22, 2021 at 1:04 PM Boris Pismenny <borisp@nvidia.com> wrote:
>>>>
>>>> From: Boris Pismenny <borisp@mellanox.com>
>>>>
>>>>
>>> ...
>>>
>>>>  };
>>>>
>>>>  const char
>>>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>>>> index e6ca5a1f3b59..4a7160bba09b 100644
>>>> --- a/net/ipv4/tcp_input.c
>>>> +++ b/net/ipv4/tcp_input.c
>>>> @@ -5149,6 +5149,9 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
>>>>                 memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
>>>>  #ifdef CONFIG_TLS_DEVICE
>>>>                 nskb->decrypted = skb->decrypted;
>>>> +#endif
>>>> +#ifdef CONFIG_ULP_DDP
>>>> +               nskb->ddp_crc = skb->ddp_crc;
>>>
>>> Probably you do not want to attempt any collapse if skb->ddp_crc is
>>> set right there.
>>>
>>
>> Right.
>>
>>>>  #endif
>>>>                 TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
>>>>                 if (list)
>>>> @@ -5182,6 +5185,11 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
>>>>  #ifdef CONFIG_TLS_DEVICE
>>>>                                 if (skb->decrypted != nskb->decrypted)
>>>>                                         goto end;
>>>> +#endif
>>>> +#ifdef CONFIG_ULP_DDP
>>>> +
>>>> +                               if (skb->ddp_crc != nskb->ddp_crc)
>>>
>>> This checks only the second, third, and remaining skbs.
>>>
>>
>> Right, as we handle the head skb above. Could you clarify?
> 
> I was simply saying you missed the first skb.
> 

But, the first SKB got handled in the change above. The code here is the
same for TLS, if it is wrong, then we already have an issue here.

>>
>>>> +                                       goto end;
>>>>  #endif
>>>>                         }
>>>>                 }
>>>
>>>
>>> tcp_collapse() is copying data from small skbs to pack it to bigger
>>> skb (one page of payload), in case
>>> of memory emergency/pressure (socket queues are full)
>>>
>>> If your changes are trying to avoid 'needless'  copies, maybe you
>>> should reconsider and let the emergency packing be done.
>>>
>>> If the copy is not _possible_, you should rephrase your changelog to
>>> clearly state the kernel _cannot_ access this memory in any way.
>>>
>>
>> The issue is that skb_condense also gets called on many skbs in
>> tcp_add_backlog and it will identify skbs that went through DDP as ideal
>> for packing, even though they are not small and packing is
>> counter-productive as data already resides in its destination.
>>
>> As mentioned above, it is possible to copy, but it is counter-productive
>> in this case. If there was a real need to access this memory, then it is
>> allowed.
> 
> Standard GRO packets from high perf drivers have no room in their
> skb->head (ie skb_tailroom() should be 0)
> 
> If you have a driver using GRO and who pulled some payload in
> skb->head, it is already too late for DDP.
> 
> So I think you are trying to add code in TCP that should not be
> needed. Perhaps mlx5 driver is doing something it should not ?
> (If this is ' copybreak'  this has been documented as being
> suboptimal, transports have better strategies)
> 
> Secondly, tcp_collapse() should absolutely not be called under regular
> workloads.
> 
> Trying to optimize this last-resort thing is a lost cause:
> If an application is dumb enough to send small packets back-to-back,
> it should be fixed (sender side has this thing called autocork, for
> applications that do not know about MSG_MORE or TC_CORK.)
> 
> (tcp_collapse is a severe source of latencies)
> 


Sorry. My response above was about skb_condense which I've confused with
tcp_collapse.

In tcp_collapse, we could allow the copy, but the problem is CRC, which
like TLS's skb->decrypted marks that the data passed the digest
validation in the NIC. If we allow collapsing SKBs with mixed marks, we
will need to force software copy+crc verification. As TCP collapse is
indeed rare and the offload is opportunistic in nature, we can make this
change and submit another version, but I'm confused; why was it OK for
TLS, while it is not OK for DDP+CRC?


  reply	other threads:[~2021-07-22 13:33 UTC|newest]

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-22 11:02 [PATCH v5 net-next 00/36] nvme-tcp receive and tarnsmit offloads Boris Pismenny
2021-07-22 11:02 ` [PATCH v5 net-next 01/36] net: Introduce direct data placement tcp offload Boris Pismenny
2021-07-22 11:26   ` Eric Dumazet
2021-07-22 12:18     ` Boris Pismenny
2021-07-22 13:10       ` Eric Dumazet
2021-07-22 13:33         ` Boris Pismenny [this message]
2021-07-22 13:39           ` Eric Dumazet
2021-07-22 14:02             ` Boris Pismenny
2021-07-22 11:02 ` [PATCH v5 net-next 02/36] iov_iter: DDP copy to iter/pages Boris Pismenny
2021-07-22 13:31   ` Christoph Hellwig
2021-07-22 20:23     ` Boris Pismenny
2021-07-23  5:03       ` Christoph Hellwig
2021-07-23  5:21         ` Al Viro
2021-08-04 14:13           ` Or Gerlitz
2021-08-10 13:29             ` Or Gerlitz
2021-07-22 20:55   ` Al Viro
2021-07-22 11:02 ` [PATCH v5 net-next 03/36] net: skb copy(+hash) iterators for DDP offloads Boris Pismenny
2021-07-22 11:02 ` [PATCH v5 net-next 04/36] net/tls: expose get_netdev_for_sock Boris Pismenny
2021-07-23  6:06   ` Christoph Hellwig
2021-08-04 13:26     ` Or Gerlitz
     [not found]       ` <20210804072918.17ba9cff@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
2021-08-04 15:07         ` Or Gerlitz
2021-08-10 13:25           ` Or Gerlitz
2021-07-22 11:02 ` [PATCH v5 net-next 05/36] nvme-tcp: Add DDP offload control path Boris Pismenny
2021-07-22 11:02 ` [PATCH v5 net-next 06/36] nvme-tcp: Add DDP data-path Boris Pismenny
2021-07-22 11:02 ` [PATCH v5 net-next 07/36] nvme-tcp: RX DDGST offload Boris Pismenny
2021-07-22 11:02 ` [PATCH v5 net-next 08/36] nvme-tcp: Deal with netdevice DOWN events Boris Pismenny
2021-07-22 11:02 ` [PATCH v5 net-next 09/36] net/mlx5: Header file changes for nvme-tcp offload Boris Pismenny
2021-07-22 11:02 ` [PATCH v5 net-next 10/36] net/mlx5: Add 128B CQE for NVMEoTCP offload Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 11/36] net/mlx5e: TCP flow steering for nvme-tcp Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 12/36] net/mlx5e: NVMEoTCP offload initialization Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 13/36] net/mlx5e: KLM UMR helper macros Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 14/36] net/mlx5e: NVMEoTCP use KLM UMRs Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 15/36] net/mlx5e: NVMEoTCP queue init/teardown Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 16/36] net/mlx5e: NVMEoTCP async ddp invalidation Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 17/36] net/mlx5e: NVMEoTCP ddp setup and resync Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 18/36] net/mlx5e: NVMEoTCP, data-path for DDP+DDGST offload Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 19/36] net/mlx5e: NVMEoTCP statistics Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 20/36] Documentation: add ULP DDP offload documentation Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 21/36] net: drop ULP DDP HW offload feature if no CSUM offload feature Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 22/36] net: Add ulp_ddp_pdu_info struct Boris Pismenny
2021-07-23 19:42   ` Sagi Grimberg
2021-07-22 11:03 ` [PATCH v5 net-next 23/36] net: Add to ulp_ddp support for fallback flow Boris Pismenny
2021-07-23  6:09   ` Christoph Hellwig
2021-07-22 11:03 ` [PATCH v5 net-next 24/36] net: Add MSG_DDP_CRC flag Boris Pismenny
2021-07-22 14:23   ` Eric Dumazet
2021-07-22 11:03 ` [PATCH v5 net-next 25/36] nvme-tcp: TX DDGST offload Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 26/36] nvme-tcp: Mapping between Tx NVMEoTCP pdu and TCP sequence Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 27/36] mlx5e: make preparation in TLS code for NVMEoTCP CRC Tx offload Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 28/36] mlx5: Add sq state test bit for nvmeotcp Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 29/36] mlx5: Add support to NETIF_F_HW_TCP_DDP_CRC_TX feature Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 30/36] net/mlx5e: NVMEoTCP DDGST TX offload TIS Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 31/36] net/mlx5e: NVMEoTCP DDGST Tx offload queue init/teardown Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 32/36] net/mlx5e: NVMEoTCP DDGST TX BSF and PSV Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 33/36] net/mlx5e: NVMEoTCP DDGST TX Data path Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 34/36] net/mlx5e: NVMEoTCP DDGST TX handle OOO packets Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 35/36] net/mlx5e: NVMEoTCP DDGST TX offload optimization Boris Pismenny
2021-07-22 11:03 ` [PATCH v5 net-next 36/36] net/mlx5e: NVMEoTCP DDGST TX statistics Boris Pismenny
2021-07-23  5:56 ` [PATCH v5 net-next 00/36] nvme-tcp receive and tarnsmit offloads Christoph Hellwig
2021-07-23 19:58   ` Sagi Grimberg
2021-08-04 13:51     ` Or Gerlitz
2021-08-06 19:46       ` Sagi Grimberg
2021-08-10 13:37         ` Or Gerlitz

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