netdev - Re: [net-next-2.6 PATCH v5 1/2] net: implement mechanism for HW based QOS

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Date:	Thu, 06 Jan 2011 19:31:30 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	John Fastabend <john.r.fastabend@...el.com>, davem@...emloft.net,
	jarkao2@...il.com, hadi@...erus.ca, shemminger@...tta.com,
	tgraf@...radead.org, nhorman@...driver.com, netdev@...r.kernel.org
Subject: Re: [net-next-2.6 PATCH v5 1/2] net: implement mechanism for HW
 based QOS

Le jeudi 06 janvier 2011 à 18:20 +0000, Ben Hutchings a écrit :
> On Tue, 2011-01-04 at 10:56 -0800, John Fastabend wrote:
> [...]
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index 0f6b1c9..ae51323 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -646,6 +646,14 @@ struct xps_dev_maps {
> >      (nr_cpu_ids * sizeof(struct xps_map *)))
> >  #endif /* CONFIG_XPS */
> >  
> > +#define TC_MAX_QUEUE	16
> > +#define TC_BITMASK	15
> > +/* HW offloaded queuing disciplines txq count and offset maps */
> > +struct netdev_tc_txq {
> > +	u16 count;
> > +	u16 offset;
> > +};
> > +
> >  /*
> >   * This structure defines the management hooks for network devices.
> >   * The following hooks can be defined; unless noted otherwise, they are
> > @@ -1146,6 +1154,9 @@ struct net_device {
> >  	/* Data Center Bridging netlink ops */
> >  	const struct dcbnl_rtnl_ops *dcbnl_ops;
> >  #endif
> > +	u8 num_tc;
> > +	struct netdev_tc_txq tc_to_txq[TC_MAX_QUEUE];
> > +	u8 prio_tc_map[TC_BITMASK+1];
> [...]
> 
> I'm still concerned by the addition of all this state to every
> net_device.  From previous discussion, Eric wanted this, citing 'false
> sharing' while Stephen thought it should be accessed indirectly.
> 
> Eric, when you refer to 'false sharing' do you mean that the TC state
> might end up sharing a cache line with some other data?  That seems
> quite unlikely as the allocation size will be 128 bytes, and it could be
> padded to fill a cache line if that's still a concern.

At the time I made a comment, the allocated data was less than 64 bytes

Problem is adding so many indirections here and here reduce latencies on
workloads handling a few packets per second.

sizeof(struct net_device)=0x600

We currently have 512 unused bytes (because of kmalloc() power of two)

(Most virtual devices have small private part added to net_device.
The real devices are probably crossing the 0x800 limit (or even 0x1000))



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