Re: [PATCH iproute2-next v3 2/2] tc: implement support for terse dump

From: Jamal Hadi Salim <jhs@mojatatu.com>
To: Vlad Buslov <vlad@buslov.dev>
Cc: Vlad Buslov <vladbu@nvidia.com>,
	dsahern@gmail.com, stephen@networkplumber.org,
	netdev@vger.kernel.org, davem@davemloft.net,
	xiyou.wangcong@gmail.com, jiri@resnulli.us, ivecera@redhat.com,
	Vlad Buslov <vladbu@mellanox.com>
Subject: Re: [PATCH iproute2-next v3 2/2] tc: implement support for terse dump
Date: Tue, 20 Oct 2020 08:29:19 -0400	[thread overview]
Message-ID: <5c79152f-1532-141a-b1d3-729fdd798b3f@mojatatu.com> (raw)
In-Reply-To: <87362a1byb.fsf@buslov.dev>

On 2020-10-19 11:18 a.m., Vlad Buslov wrote:
> 
> On Mon 19 Oct 2020 at 16:48, Jamal Hadi Salim <jhs@mojatatu.com> wrote:
>> On 2020-10-18 8:16 a.m., Vlad Buslov wrote:

[..]

>> That could be a good thing, no? you get to see the action name with the
>> error. Its really not a big deal if you decide to do a->terse_print()
>> instead.
> 
> Maybe. Just saying that this change would also change user-visible
> iproute2 behavior.
> 

You are right(for the non-terse output). tbh, not sure if it is a big
deal given it happens only for the error case (where scripts look
for exit codes typically); having said that:
a ->terse_print() would be ok

> It is not a trivial change. To get this data we need to call
> tc_action_ops->dump() which puts bunch of other unrelated info in
> TCA_OPTIONS nested attr. This hurts both dump size and runtime
> performance. Even if we add another argument to dump "terse dump, print
> only index", index is still part of larger options structure which
> includes at least following fields:
> 
> #define tc_gen \
> 	__u32                 index; \
> 	__u32                 capab; \
> 	int                   action; \
> 	int                   refcnt; \
> 	int                   bindcnt
>

index is the _only_ important field for analytics purposes in that list.
i.e if i know the index i can correlate stats with one or more
filters (whether shared or not).
My worry is you have a very specific use case for your hardware or
maybe it is ovs - where counters are uniquely tied to filters and
there is no sharing. And possibly maybe only one counter can be tied
to a filter (was not sure if you could handle more than one action
in the terse from looking at the code).
Our assumptions so far had no such constraints.
Maybe a new TERSE_OPTIONS TLV, and then add an extra flag
to indicate interest in the tlv? Peharps store the stats in it as well.

> This wouldn't be much of a terse dump anymore. What prevents user that
> needs all action info from calling regular dump? It is not like terse
> dump substitutes it or somehow makes it harder to use.

Both scaling and correctness are important. You have the cookie
in the terse dump, thats a lot of data.
In our case we totally bypass filters to reduce the amount of data
crossing to user space (tc action ls). Theres still a lot of data
crossing which we could trim with a terse dump. All we are interested
in are stats. Another alternative is perhaps to introduce the index for
the direct dump.

cheers,
jamal