netdev - Re: [PATCH v1 13/15] io_uring/zcrx: add copy fallback

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Message-ID: <6b57fb43-1271-4487-9342-5f82c972b9c5@davidwei.uk>
Date: Tue, 8 Oct 2024 09:40:58 -0700
From: David Wei <dw@...idwei.uk>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: io-uring@...r.kernel.org, netdev@...r.kernel.org,
 Jens Axboe <axboe@...nel.dk>, Pavel Begunkov <asml.silence@...il.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jesper Dangaard Brouer <hawk@...nel.org>, David Ahern <dsahern@...nel.org>,
 Mina Almasry <almasrymina@...gle.com>, David Wei <dw@...idwei.uk>
Subject: Re: [PATCH v1 13/15] io_uring/zcrx: add copy fallback

On 2024-10-08 08:58, Stanislav Fomichev wrote:
> On 10/07, David Wei wrote:
>> From: Pavel Begunkov <asml.silence@...il.com>
>>
>> There are scenarios in which the zerocopy path might get a normal
>> in-kernel buffer, it could be a mis-steered packet or simply the linear
>> part of an skb. Another use case is to allow the driver to allocate
>> kernel pages when it's out of zc buffers, which makes it more resilient
>> to spikes in load and allow the user to choose the balance between the
>> amount of memory provided and performance.
> 
> Tangential: should there be some clear way for the users to discover that
> (some counter of some entry on cq about copy fallback)?
> 
> Or the expectation is that somebody will run bpftrace to diagnose
> (supposedly) poor ZC performance when it falls back to copy?

Yeah there definitely needs to be a way to notify the user that copy
fallback happened. Right now I'm relying on bpftrace hooking into
io_zcrx_copy_chunk(). Doing it per cqe (which is emitted per frag) is
too much. I can think of two other options:

1. Send a final cqe at the end of a number of frag cqes with a count of
   the number of copies.
2. Register a secondary area just for handling copies.

Other suggestions are also very welcome.