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

[Joe Damato <jdamato@fastly.com>]

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@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?
> 
> 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 ?