netdev - Re: [PATCH net-next v4 1/2] net sched actions: dump more than TCA_ACT_MAX

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Date:   Wed, 19 Apr 2017 18:17:36 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org,
        eric.dumazet@...il.com, xiyou.wangcong@...il.com
Subject: Re: [PATCH net-next v4 1/2] net sched actions: dump more than
 TCA_ACT_MAX_PRIO actions per batch

Wed, Apr 19, 2017 at 06:07:48PM CEST, jhs@...atatu.com wrote:
>On 17-04-19 11:54 AM, Jiri Pirko wrote:
>> Wed, Apr 19, 2017 at 05:37:15PM CEST, jhs@...atatu.com wrote:
>> > On 17-04-19 09:13 AM, Jiri Pirko wrote:
>> > > Wed, Apr 19, 2017 at 03:03:59PM CEST, jhs@...atatu.com wrote:
>> > > > On 17-04-19 08:36 AM, Jiri Pirko wrote:
>> > > > > Wed, Apr 19, 2017 at 01:57:29PM CEST, jhs@...atatu.com wrote:
>> > > > > > From: Jamal Hadi Salim <jhs@...atatu.com>
>> > > > 
>> > > > > > include/uapi/linux/rtnetlink.h | 21 +++++++++++++++++++--
>> > > > > > net/sched/act_api.c            | 43 ++++++++++++++++++++++++++++++++----------
>> > > > > > 3 files changed, 53 insertions(+), 12 deletions(-)
>> > 
>> > > > > > +#define TCAA_MAX (__TCAA_MAX - 1)
>> > > > > > #define TA_RTA(r)  ((struct rtattr*)(((char*)(r)) + NLMSG_ALIGN(sizeof(struct tcamsg))))
>> > > > > > #define TA_PAYLOAD(n) NLMSG_PAYLOAD(n,sizeof(struct tcamsg))
>> > > > > > -#define TCA_ACT_TAB 1 /* attr type must be >=1 */	
>> > > > > > -#define TCAA_MAX 1
>> > > > > > +#define TCA_ACT_TAB TCAA_ACT_TAB
>> > > > > 
>> > > > > This is mess. What does "TCAA" stand for?
>> > > > 
>> > > > TC Actions Attributes.  What would you call it? I could have
>> > > > called it TCA_ROOT etc. But maybe a comment to just call it
>> > > > TC Actions Attributes would be enough?
>> > > 
>> > > TCA_DUMP_X
>> > > 
>> > > it is only for dumping. Naming it "attribute" seems weird. Same as if
>> > > you have: int variable_something;
>> > > 
>> > 
>> > Jiri, this is not just for dumping. We are describing high level
>> > attributes for tc actions.
>> 
>> This is already present:
>> enum {
>>         TCA_ACT_UNSPEC,
>>         TCA_ACT_KIND,
>>         TCA_ACT_OPTIONS,
>>         TCA_ACT_INDEX,
>>         TCA_ACT_STATS,
>>         TCA_ACT_PAD,
>>         TCA_ACT_COOKIE,
>>         __TCA_ACT_MAX
>> };
>> 
>> This is nested inside the dump message (TCAA_MAX, TCA_ACT_TAB)
>> 
>> Looks like you are mixing these 2.
>> 
>
>No - That space is per action. The space i am defining is
>above that in the the hierarchy. There used to be only
>one attribute there (TCA_ACT_TAB) and now we are making
>it more generic.

Right. That's what I say. And that higher level is used only for
dumping. That's why I suggested TCA_ACT_DUMP prefix.


>
>> 
>> 
>
>> > It is a _lot_ of code to change! Note:
>> > This is all the UAPI visible code (the same coding style for 20 years).
>> > I am worried about this part.
>> 
>> We'll see. Lets do it in a sensitive way, in steps. But for new things,
>> I think it is good not to stick with old and outlived habits.
>> 
>> 
>
>I know you have the muscle to get it done - so fine, i will start
>with this one change.
>
>> 
>> Netlink is TLV, should be used as TLV. I don't see how you can run out
>> any space. You tend to use Netlink in some weird hybrid mode, with only
>> argument being space. I think that couple of bytes wasted is not
>> a problem at all...
>> 
>
>You are not making sense to me still.
>What you describe as "a couple of bytes" adds up when you have
>a large number of objects. I am trying to cut down on data back
>and forth from user/kernel and a bitmap is a well understood entity.

The attributes in question are per-message, not per-object


>
>Even if i did use a TLV - when i am representing flags which require one
>bit each - it makes sense to use a bitmask encapsulated in a TLV. Not
>to waste a TLV per bit.

That is the only correct way. For example, in future the kernel may
report in extended ack that it does not support some specific attribute
user passed. If you pack it all in one attr, that would not be possible.
Also, what prevents user from passing random data on bit flag positions
that you are not using? Current kernel would ignore it. Next kernel will
do something different according to the flag bit. That is UAPI break.
Essentialy the same thing what DaveM said about the struct. You have to
define this completely, at the beginning, not possible to use the unused
bits for other things in the future.