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

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Message-Id: <20141106.223603.678182467467024928.davem@davemloft.net>
Date:	Thu, 06 Nov 2014 22:36:03 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
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

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 06 Nov 2014 14:11:20 -0800

> 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 ?

I think for such a critical path in the kernel it's worth it to avoid
these increments for every packet, just to compute a value that's
sitting in a register already in the driver's poll routine.

Eric, you've been trimming cpu IRQ disables from these exact code
paths, you should know better than me :-)

I'm willing to do some of the monkey work of converting as many
drivers as can be trivially done if you want.  Almost all of the
ones I looked at have the work_done variable right there at the
napi_complete() call site.

The rest can stay unconverted and not get access to this new facility.
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