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Message-ID: <20160224061128.GB12134@vergenet.net>
Date: Wed, 24 Feb 2016 15:11:31 +0900
From: Simon Horman <simon.horman@...ronome.com>
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>
[snip]
> diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> index f766bcb..c509fc8 100644
> --- a/net/sched/cls_u32.c
> +++ b/net/sched/cls_u32.c
> @@ -59,6 +59,7 @@ struct tc_u_knode {
> #ifdef CONFIG_CLS_U32_PERF
> struct tc_u32_pcnt __percpu *pf;
> #endif
> + u32 flags;
> #ifdef CONFIG_CLS_U32_MARK
> u32 val;
> u32 mask;
> @@ -425,12 +426,18 @@ static int u32_delete_key(struct tcf_proto *tp, struct tc_u_knode *key)
> return 0;
> }
>
> -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;
> }
>
> static void u32_remove_hw_knode(struct tcf_proto *tp, u32 handle)
> @@ -442,7 +449,7 @@ static void u32_remove_hw_knode(struct tcf_proto *tp, u32 handle)
> offload.type = TC_SETUP_CLSU32;
> offload.cls_u32 = &u32_offload;
>
> - if (u32_should_offload(dev)) {
> + if (u32_should_offload(dev, 0)) {
> offload.cls_u32->command = TC_CLSU32_DELETE_KNODE;
> offload.cls_u32->knode.handle = handle;
> dev->netdev_ops->ndo_setup_tc(dev, tp->q->handle,
Here a request is made to delete the classifier from hardware regardless
of if TCA_U32_FLAGS_SOFTWARE is set or not. This seems sensible to me.
> @@ -450,7 +457,9 @@ static void u32_remove_hw_knode(struct tcf_proto *tp, u32 handle)
> }
> }
>
> -static void u32_replace_hw_hnode(struct tcf_proto *tp, struct tc_u_hnode *h)
> +static void u32_replace_hw_hnode(struct tcf_proto *tp,
> + struct tc_u_hnode *h,
> + u32 flags)
> {
> struct net_device *dev = tp->q->dev_queue->dev;
> struct tc_cls_u32_offload u32_offload = {0};
> @@ -459,7 +468,7 @@ static void u32_replace_hw_hnode(struct tcf_proto *tp, struct tc_u_hnode *h)
> offload.type = TC_SETUP_CLSU32;
> offload.cls_u32 = &u32_offload;
>
> - if (u32_should_offload(dev)) {
> + if (u32_should_offload(dev, flags)) {
> offload.cls_u32->command = TC_CLSU32_NEW_HNODE;
> offload.cls_u32->hnode.divisor = h->divisor;
> offload.cls_u32->hnode.handle = h->handle;
But here an update is only made if flag is TCA_U32_FLAGS_SOFTWARE.
I wonder if this means we can get into a situation where the classifier
present in software and hardware differ. Something like this.
1. classifier is added to software and hardware (TCA_U32_FLAGS_SOFTWARE is set)
2. classifier is updated in software only (TCA_U32_FLAGS_SOFTWARE is not set)
> @@ -479,7 +488,7 @@ static void u32_clear_hw_hnode(struct tcf_proto *tp, struct tc_u_hnode *h)
> offload.type = TC_SETUP_CLSU32;
> offload.cls_u32 = &u32_offload;
>
> - if (u32_should_offload(dev)) {
> + if (u32_should_offload(dev, 0)) {
> offload.cls_u32->command = TC_CLSU32_DELETE_HNODE;
> offload.cls_u32->hnode.divisor = h->divisor;
> offload.cls_u32->hnode.handle = h->handle;
> @@ -490,7 +499,9 @@ static void u32_clear_hw_hnode(struct tcf_proto *tp, struct tc_u_hnode *h)
> }
> }
>
> -static void u32_replace_hw_knode(struct tcf_proto *tp, struct tc_u_knode *n)
> +static void u32_replace_hw_knode(struct tcf_proto *tp,
> + struct tc_u_knode *n,
> + u32 flags)
> {
> struct net_device *dev = tp->q->dev_queue->dev;
> struct tc_cls_u32_offload u32_offload = {0};
> @@ -499,7 +510,7 @@ static void u32_replace_hw_knode(struct tcf_proto *tp, struct tc_u_knode *n)
> offload.type = TC_SETUP_CLSU32;
> offload.cls_u32 = &u32_offload;
>
> - if (u32_should_offload(dev)) {
> + if (u32_should_offload(dev, flags)) {
> offload.cls_u32->command = TC_CLSU32_REPLACE_KNODE;
> offload.cls_u32->knode.handle = n->handle;
> offload.cls_u32->knode.fshift = n->fshift;
[snip]
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