netdev - Re: [RFC] GRO scalability

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Date:	Sat, 06 Oct 2012 08:22:21 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Jesse Gross <jesse@...ira.com>
Subject: Re: [RFC] GRO scalability

On Sat, 2012-10-06 at 13:14 +0800, Herbert Xu wrote:
> On Sat, Oct 06, 2012 at 07:08:46AM +0200, Eric Dumazet wrote:
> > Le samedi 06 octobre 2012 à 12:11 +0800, Herbert Xu a écrit :
> > > On Fri, Oct 05, 2012 at 04:52:27PM +0200, Eric Dumazet wrote:
> > > > Current GRO cell is somewhat limited :
> > > > 
> > > > - It uses a single list (napi->gro_list) of pending skbs
> > > > 
> > > > - This list has a limit of 8 skbs (MAX_GRO_SKBS)
> > > > 
> > > > - Workloads with lot of concurrent flows have small GRO hit rate but
> > > >   pay high overhead (in inet_gro_receive())
> > > > 
> > > > - Increasing MAX_GRO_SKBS is not an option, because GRO
> > > >   overhead becomes too high.
> > > 
> > > Yeah these were all meant to be addressed at some point.
> > > 
> > > > - Packets can stay a long time held in GRO cell (there is
> > > >   no flush if napi never completes on a stressed cpu)
> > > 
> > > This should never happen though.  NAPI runs must always be
> > > punctuated just to guarantee one card never hogs a CPU.  Which
> > > driver causes these behaviour?
> > 
> > I believe its a generic issue, not specific to a driver.
> > 
> > napi_gro_flush() is only called from napi_complete() 
> > 
> > Some drivers (marvell/skge.c & realtek/8139cp.c) calls it only because
> > they 'inline' napi_complete()
> 
> So which driver has the potential of never doing napi_gro_flush?

All drivers.

If the napi->poll() consumes all budget, we dont call napi_complete()



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