netdev - Re: [PATCH net-next] net/sched: store the last executed chain also for clsact egress

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Message-ID: <YQRSlETLgowgik2x@dcaratti.users.ipa.redhat.com>
Date:   Fri, 30 Jul 2021 21:27:16 +0200
From:   Davide Caratti <dcaratti@...hat.com>
To:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net/sched: store the last executed chain also
 for clsact egress

hello Cong, thanks for looking at this!

On Wed, Jul 28, 2021 at 03:16:02PM -0700, Cong Wang wrote:
> On Wed, Jul 28, 2021 at 11:40 AM Davide Caratti <dcaratti@...hat.com> wrote:
> >
> > currently, only 'ingress' and 'clsact ingress' qdiscs store the tc 'chain
> > id' in the skb extension. However, userspace programs (like ovs) are able
> > to setup egress rules, and datapath gets confused in case it doesn't find
> > the 'chain id' for a packet that's "recirculated" by tc.
> > Change tcf_classify() to have the same semantic as tcf_classify_ingress()
> > so that a single function can be called in ingress / egress, using the tc
> > ingress / egress block respectively.
> 
> I wonder if there is any performance impact with this change? As
> tcf_classify() now may allocate skb ext (tc_skb_ext_alloc()) too,
> right after __tcf_classify().
> 
> Thanks.

I think there is some performance drop for users that activate
TC_SKB_EXT, in case packet doesn't match the filter *and* last
executed_chain is non-zero. But in this case, I also think it's a good
choice to spend some cycles to save the chain id in the extension: I
might be wrong, but AFAIK openvswitch is the only "user" that configures
TC in this way.

Do you have in mind a specific case where performance can be degraded
because of this commit? if so, I can try to investigate more.

thanks!
-- 
davide