netdev - Re: [net-next PATCH 3/4] net: sched: cls_u32 add bit to specify software only rules

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Message-ID: <20160224080440.GA26500@office.Home>
Date:	Wed, 24 Feb 2016 10:04:40 +0200
From:	"Amir Vadai\"" <amir@...ai.me>
To:	John Fastabend <john.fastabend@...il.com>
Cc:	jiri@...nulli.us, daniel@...earbox.net, netdev@...r.kernel.org,
	alexei.starovoitov@...il.com, davem@...emloft.net, jhs@...atatu.com
Subject: Re: [net-next PATCH 3/4] net: sched: cls_u32 add bit to specify
 software only rules

On Tue, Feb 23, 2016 at 11:03:21AM -0800, John Fastabend wrote:
> In the initial implementation the only way to stop a rule from being
> inserted into the hardware table was via the device feature flag.
> However this doesn't work well when working on an end host system
> where packets are expect to hit both the hardware and software
> datapaths.
> 
> For example we can imagine a rule that will match an IP address and
> increment a field. If we install this rule in both hardware and
> software we may increment the field twice. To date we have only
> added support for the drop action so we have been able to ignore
> these cases. But as we extend the action support we will hit this
> example plus more such cases. Arguably these are not even corner
> cases in many working systems these cases will be common.
> 
> To avoid forcing the driver to always abort (i.e. the above example)
> this patch adds a flag to add a rule in software only. A careful
> user can use this flag to build software and hardware datapaths
> that work together. One example we have found particularly useful
> is to use hardware resources to set the skb->mark on the skb when
> the match may be expensive to run in software but a mark lookup
> in a hash table is cheap. The idea here is hardware can do in one
> lookup what the u32 classifier may need to traverse multiple lists
> and hash tables to compute. The flag is only passed down on inserts
> on deletion to avoid stale references in hardware we always try
> to remove a rule if it exists.
> 
> Notice we do not add a hardware only case here. If you were to
> add a hardware only case then you are stuck with the problem
> of where to stick the software representation of that filter
> rule. If its stuck on the same filter list as the software only and
> software/hardware rules it then has to be walked over and ignored
> in the classify path. The overhead is not huge but is measurable.
> And with so much work being invested in speeding up rx/tx of
> pkt processing this is unacceptable IMO. The other option is to
> have a special hook just for hardware only resources. This is
> implemented in the next patch.
> 
> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>

[...]

>  
> -static bool u32_should_offload(struct net_device *dev)
> +static bool u32_should_offload(struct net_device *dev, u32 flags)
>  {
>  	if (!(dev->features & NETIF_F_HW_TC))
>  		return false;
>  
> -	return dev->netdev_ops->ndo_setup_tc;
> +	if (flags & TCA_U32_FLAGS_SOFTWARE)
> +		return false;
> +
> +	if (!dev->netdev_ops->ndo_setup_tc)
> +		return false;
> +
> +	return true;
>  }
This function and flag should be a generic filter attribute - not just
u32.

Thanks,
Amir

[...]