netdev - Re: [PATCH net-next v8 08/17] net: add helper executing custom callback from napi

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Message-ID: <601572a4-f5a8-4dec-a07d-0c035c4fbb79@gmail.com>
Date: Tue, 10 Dec 2024 04:11:31 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Jakub Kicinski <kuba@...nel.org>, David Wei <dw@...idwei.uk>
Cc: io-uring@...r.kernel.org, netdev@...r.kernel.org,
 Jens Axboe <axboe@...nel.dk>, 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>,
 Stanislav Fomichev <stfomichev@...il.com>, Joe Damato <jdamato@...tly.com>,
 Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH net-next v8 08/17] net: add helper executing custom
 callback from napi

On 12/10/24 03:44, Jakub Kicinski wrote:
> On Wed,  4 Dec 2024 09:21:47 -0800 David Wei wrote:
>> It's useful to have napi private bits and pieces like page pool's fast
>> allocating cache, so that the hot allocation path doesn't have to do any
>> additional synchronisation. In case of io_uring memory provider
>> introduced in following patches, we keep the consumer end of the
>> io_uring's refill queue private to napi as it's a hot path.
>>
>> However, from time to time we need to synchronise with the napi, for
>> example to add more user memory or allocate fallback buffers. Add a
>> helper function napi_execute that allows to run a custom callback from
>> under napi context so that it can access and modify napi protected
>> parts of io_uring. It works similar to busy polling and stops napi from
>> running in the meantime, so it's supposed to be a slow control path.
> 
> Let's leave this out, please. I seriously doubt this works reliably
> and the bar for adding complexity to NAPI is fairly high. A commit
> which doesn't even quote any perf numbers will certainly not clear it.

I reworked it and got rid of the patch, though I don't see why
performance here would matter. It's a very slow path if all
paths failed, batching inside wasn't done right on the io_uring
for simplicity, but that was certainly planned a follow up.

-- 
Pavel Begunkov