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

[Joe Damato <jdamato@fastly.com>]

On Mon, Oct 07, 2024 at 03:15:56PM -0700, David Wei wrote:
> From: Pavel Begunkov <asml.silence@gmail.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@gmail.com>
> Signed-off-by: David Wei <dw@davidwei.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?