netdev - Re: [PATCH v1 08/15] net: add helper executing custom callback from napi

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Message-ID: <8fcb5c94-959e-4daa-836b-ab14e25c3a89@gmail.com>
Date: Wed, 9 Oct 2024 20:12:13 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Joe Damato <jdamato@...tly.com>, David Wei <dw@...idwei.uk>,
 io-uring@...r.kernel.org, netdev@...r.kernel.org,
 Jens Axboe <axboe@...nel.dk>, 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 08/15] net: add helper executing custom callback from
 napi

On 10/9/24 17:13, Joe Damato wrote:
> On Wed, Oct 09, 2024 at 04:09:53PM +0100, Pavel Begunkov wrote:
>> On 10/8/24 23:25, Joe Damato wrote:
>>> On Mon, Oct 07, 2024 at 03:15:56PM -0700, David Wei wrote:
>>>> From: Pavel Begunkov <asml.silence@...il.com>
>>>
>>> [...]
>>>
>>>> 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.
>>>>
>>>> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
>>>> Signed-off-by: David Wei <dw@...idwei.uk>
>>>
>>> [...]
>>>
>>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>>> index 1e740faf9e78..ba2f43cf5517 100644
>>>> --- a/net/core/dev.c
>>>> +++ b/net/core/dev.c
>>>> @@ -6497,6 +6497,59 @@ void napi_busy_loop(unsigned int napi_id,
>>>>    }
>>>>    EXPORT_SYMBOL(napi_busy_loop);
>>>> +void napi_execute(unsigned napi_id,
>>>> +		  void (*cb)(void *), void *cb_arg)
>>>> +{
>>>> +	struct napi_struct *napi;
>>>> +	bool done = false;
>>>> +	unsigned long val;
>>>> +	void *have_poll_lock = NULL;
>>>> +
>>>> +	rcu_read_lock();
>>>> +
>>>> +	napi = napi_by_id(napi_id);
>>>> +	if (!napi) {
>>>> +		rcu_read_unlock();
>>>> +		return;
>>>> +	}
>>>> +
>>>> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
>>>> +		preempt_disable();
>>>> +	for (;;) {
>>>> +		local_bh_disable();
>>>> +		val = READ_ONCE(napi->state);
>>>> +
>>>> +		/* If multiple threads are competing for this napi,
>>>> +		* we avoid dirtying napi->state as much as we can.
>>>> +		*/
>>>> +		if (val & (NAPIF_STATE_DISABLE | NAPIF_STATE_SCHED |
>>>> +			  NAPIF_STATE_IN_BUSY_POLL))
>>>> +			goto restart;
>>>> +
>>>> +		if (cmpxchg(&napi->state, val,
>>>> +			   val | NAPIF_STATE_IN_BUSY_POLL |
>>>> +				 NAPIF_STATE_SCHED) != val)
>>>> +			goto restart;
>>>> +
>>>> +		have_poll_lock = netpoll_poll_lock(napi);
>>>> +		cb(cb_arg);
>>>
>>> A lot of the above code seems quite similar to __napi_busy_loop, as
>>> you mentioned.
>>>
>>> It might be too painful, but I can't help but wonder if there's a
>>> way to refactor this to use common helpers or something?
>>>
>>> I had been thinking that the napi->state check /
>>> cmpxchg could maybe be refactored to avoid being repeated in both
>>> places?
>>
>> Yep, I can add a helper for that, but I'm not sure how to
>> deduplicate it further while trying not to pollute the
>> napi polling path.
> 
> It was just a minor nit; I wouldn't want to hold back this important
> work just for that.
> 
> I'm still looking at the code myself to see if I can see a better
> arrangement of the code.
> 
> But that could always come later as a cleanup for -next ?

It's still early, there will be a v6 anyway. And thanks for
taking a look.

-- 
Pavel Begunkov