netdev - Re: [PATCH net-next] net: gro: add a per device gro flush timer

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Date:	Thu, 06 Nov 2014 14:11:20 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, ogerlitz@...lanox.com, willemb@...gle.com
Subject: Re: [PATCH net-next] net: gro: add a per device gro flush timer

On Thu, 2014-11-06 at 16:25 -0500, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Wed, 05 Nov 2014 16:55:20 -0800
> 
> > @@ -4430,8 +4432,19 @@ void napi_complete(struct napi_struct *n)
> >  	if (unlikely(test_bit(NAPI_STATE_NPSVC, &n->state)))
> >  		return;
> >  
> > -	napi_gro_flush(n, false);
> > +	if (n->gro_list) {
> > +		unsigned long timeout = 0;
> > +
> > +		if (n->napi_rx_count)
> > +			timeout = n->dev->gro_flush_timeout;
> 
> Under what circumstances would we see n->gro_list non-NULL yet
> n->napi_rx_count == 0?
> 
> I'm not so sure it can happen.
> 
> Said another way, it looks to me like you could implement this
> using less state.

My goal was to not change any driver, doing a generic change.

Drivers call napi_complete() in their rx napi handler without giving us
the 'work_done' value which tells us if a packet was processed.

So I added a counter that is increased for every packet given to GRO
engine (napi_rx_count), so that napi_complete() has a clue if a packet
was received in _this_ NAPI run.

If at least one packet was received (and we still have packets in
gro_list) -> We ream the 'timer'
If not, we flush packets in GRO engine.

In order to avoid this state, I would have to add a new method, like
napi_complete_done(napi, work_done), and change drivers. I am not sure
its worth the effort ?

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