netdev - Re: [PATCH v1 10/15] io_uring/zcrx: add io_zcrx

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Message-ID: <8ead850d-3374-43e0-a7b0-c9cd031596ed@kernel.dk>
Date: Wed, 9 Oct 2024 13:06:44 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Pavel Begunkov <asml.silence@...il.com>, David Wei <dw@...idwei.uk>,
 io-uring@...r.kernel.org, netdev@...r.kernel.org
Cc: 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>
Subject: Re: [PATCH v1 10/15] io_uring/zcrx: add io_zcrx_area

On 10/9/24 1:05 PM, Pavel Begunkov wrote:
> On 10/9/24 19:02, Jens Axboe wrote:
>> On 10/7/24 4:15 PM, David Wei wrote:
> ...
>>> +struct io_zcrx_area {
>>> +    struct net_iov_area    nia;
>>> +    struct io_zcrx_ifq    *ifq;
>>> +
>>> +    u16            area_id;
>>> +    struct page        **pages;
>>> +
>>> +    /* freelist */
>>> +    spinlock_t        freelist_lock ____cacheline_aligned_in_smp;
>>> +    u32            free_count;
>>> +    u32            *freelist;
>>> +};
>>
>> I'm wondering if this really needs an aligned lock? Since it's only a
>> single structure, probably not a big deal. But unless there's evidence
>> to the contrary, might not be a bad idea to just kill that.
> 
> napi and IORING_OP_RECV_ZC can run on different CPUs, I wouldn't
> want the fields before the lock being contended by the lock
> because of cache line sharing, would especially hurt until it's
> warmed up well. Not really profiled, but not like we need to
> care about space here.

Right, as mentioned it's just a single struct, so doesn't matter that
much. I guess my testing all ran with same cpu for napi + rx, so would
not have seen it regardless. We can keep it as-is.

-- 
Jens Axboe