Re: [PATCH net-next 1/3] flow_offload: allow user to offload tc action to net device

[Vlad Buslov <vladbu@nvidia.com>]

On Tue 27 Jul 2021 at 16:04, Simon Horman <simon.horman@corigine.com> wrote:
> On Thu, Jul 22, 2021 at 09:33:09AM -0400, Jamal Hadi Salim wrote:
>> On 2021-07-22 9:29 a.m., Vlad Buslov wrote:
>> > On Thu 22 Jul 2021 at 12:19, Simon Horman <simon.horman@corigine.com> wrote:
>> > > From: Baowen Zheng <baowen.zheng@corigine.com>
>> > > 
>> > > Use flow_indr_dev_register/flow_indr_dev_setup_offload to
>> > > offload tc action.
>> > > 
>> > > We offload the tc action mainly for ovs meter configuration.
>> > > Make some basic changes for different vendors to return EOPNOTSUPP.
>> > > 
>> > > We need to call tc_cleanup_flow_action to clean up tc action entry since
>> > > in tc_setup_action, some actions may hold dev refcnt, especially the mirror
>> > > action.
>> > > 
>> > > As per review from the RFC, the kernel test robot will fail to run, so
>> > > we add CONFIG_NET_CLS_ACT control for the action offload.
>> > > 
>> > > Signed-off-by: Baowen Zheng <baowen.zheng@corigine.com>
>> > > Signed-off-by: Louis Peens <louis.peens@corigine.com>
>> > > Signed-off-by: Simon Horman <simon.horman@corigine.com>
>> > > ---
>> > >   drivers/net/ethernet/broadcom/bnxt/bnxt_tc.c  |  2 +-
>> > >   .../ethernet/mellanox/mlx5/core/en/rep/tc.c   |  3 ++
>> 
>> > >   			    void *data,
>> > >   			    void (*cleanup)(struct flow_block_cb *block_cb))
>> > >   {
>> > > +	if (!netdev)
>> > > +		return -EOPNOTSUPP;
>> > > +
>> > >   	switch (type) {
>> > >   	case TC_SETUP_BLOCK:
>> > >   		return mlx5e_rep_indr_setup_block(netdev, sch, cb_priv, type_data,
>> > > diff --git a/drivers/net/ethernet/netronome/nfp/flower/offload.c b/drivers/net/ethernet/netronome/nfp/flower/offload.c
>> 
>> [..]
>> 
>> > > +	/* offload actions to hardware if possible */
>> > > +	tcf_action_offload_cmd(actions, extack);
>> > > +
>> > 
>> > I think this has already been suggested for RFC, but some sort of
>> > visibility for offload status of action would be extremely welcome.
>> > Perhaps "IN_HW" flag and counter, similar to what we have for offloaded
>> > filters.
>> > 
>> 
>> Also showing a tc command line in the cover letter on how one would
>> ask for a specific action to be offloaded.
>
> In practice actions are offloaded when a flow using them is offloaded.
> So I think we need to consider what the meaning of IN_HW is.
>
> Is it that:
>
> * The driver (and potentially hardware, though not in our current
>   implementation) has accepted the action for offload;
> * That a classifier that uses the action has bee offloaded;
> * Or something else?

I think we have the same issue with filters - they might not be in
hardware after driver callback returned "success" (due to neigh state
being invalid for tunnel_key encap, for example).

>
> With regards to a counter, I'm not quite sure what this would be:
>
> * The number of devices where the action has been offloaded (which ties
>   into the question of what we mean by IN_HW)
> * The number of offloaded classifier instances using the action
> * Something else

I would prefer to have semantics similar to filters:

1. Count number of driver callbacks that returned "success".

2. If count > 0, then set in_hw flag.

3. Set in_hw_count to success count.

This would allow user to immediately determine whether action passed
driver validation.

>
> Regarding a flag to control offload:
>
> * For classifiers (at least the flower classifier) there is the skip_sw and
>   skip_hw flags, which allow control of placement of a classifier in SW and
>   HW.
> * We could add similar flags for actions, which at least in my
>   world view would have the net-effect of controlling which classifiers can
>   be added to sw and hw - f.e. a classifier that uses an action marked
>   skip_hw could not be added to HW.
> * Doing so would add some extra complexity and its not immediately apparent
>   to me what the use-case would be given that there are already flags for
>   classifiers.

Yeah, adding such flag for action offload seems to complicate things.
Also, "skip_sw" flag doesn't even make much sense for actions. I thought
that "skip_hw" flag would be nice to have for users that would like to
avoid "spamming" their NIC drivers (potentially causing higher latency
and resource consumption) for filters/actions they have no intention to
offload to hardware, but I'm not sure how useful is that option really
is.