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

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Message-ID: <32ca8ddf-2116-43b9-b434-d8393cdbdde1@davidwei.uk>
Date: Mon, 21 Oct 2024 11:31:42 -0700
From: David Wei <dw@...idwei.uk>
To: Paolo Abeni <pabeni@...hat.com>, 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 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().

> 
> Thanks,
> 
> Paolo
>