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

[Pavel Begunkov <asml.silence@gmail.com>]

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.

-- 
Pavel Begunkov