netdev - Re: [PATCH v6 14/15] io_uring/zcrx: add copy fallback

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Message-ID: <6c1e7d85-cfd3-4525-9f0b-5a88c3538286@redhat.com>
Date: Tue, 22 Oct 2024 09:48:52 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: David Wei <dw@...idwei.uk>, io-uring@...r.kernel.org,
 netdev@...r.kernel.org
Cc: Jens Axboe <axboe@...nel.dk>, Pavel Begunkov <asml.silence@...il.com>,
 Jakub Kicinski <kuba@...nel.org>, "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 v6 14/15] io_uring/zcrx: add copy fallback

On 10/21/24 20:31, David Wei wrote:
> On 2024-10-21 07:40, Paolo Abeni wrote:
>> On 10/16/24 20:52, David Wei wrote:
>>> @@ -540,6 +562,34 @@ static const struct memory_provider_ops io_uring_pp_zc_ops = {
>>>  	.scrub			= io_pp_zc_scrub,
>>>  };
>>>  
>>> +static void io_napi_refill(void *data)
>>> +{
>>> +	struct io_zc_refill_data *rd = data;
>>> +	struct io_zcrx_ifq *ifq = rd->ifq;
>>> +	netmem_ref netmem;
>>> +
>>> +	if (WARN_ON_ONCE(!ifq->pp))
>>> +		return;
>>> +
>>> +	netmem = page_pool_alloc_netmem(ifq->pp, GFP_ATOMIC | __GFP_NOWARN);
>>> +	if (!netmem)
>>> +		return;
>>> +	if (WARN_ON_ONCE(!netmem_is_net_iov(netmem)))
>>> +		return;
>>> +
>>> +	rd->niov = netmem_to_net_iov(netmem);
>>> +}
>>> +
>>> +static struct net_iov *io_zc_get_buf_task_safe(struct io_zcrx_ifq *ifq)
>>> +{
>>> +	struct io_zc_refill_data rd = {
>>> +		.ifq = ifq,
>>> +	};
>>> +
>>> +	napi_execute(ifq->napi_id, io_napi_refill, &rd);
>>
>> Under UDP flood the above has unbounded/unlimited execution time, unless
>> you set NAPI_STATE_PREFER_BUSY_POLL. Is the allocation schema here
>> somehow preventing such unlimited wait?
> 
> Hi Paolo. Do you mean that under UDP flood, napi_execute() will have
> unbounded execution time because napi_state_start_busy_polling() and
> need_resched() will always return false? My understanding is that
> need_resched() will eventually kick the caller task out of
> napi_execute().

Sorry for the short reply. Let's try to consolidate this discussion on
patch 8, which is strictly related had has the relevant code more handy.

Thanks,

Paolo