netdev - Re: [iproute PATCH] tc: u32: Fix key folding in sample option

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Date:   Thu, 4 Feb 2021 10:28:26 -0500
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Phil Sutter <phil@....cc>,
        Stephen Hemminger <stephen@...workplumber.org>,
        netdev@...r.kernel.org
Subject: Re: [iproute PATCH] tc: u32: Fix key folding in sample option

On 2021-02-04 9:50 a.m., Phil Sutter wrote:
> On Thu, Feb 04, 2021 at 09:34:01AM -0500, Jamal Hadi Salim wrote:
>> On 2021-02-04 9:04 a.m., Phil Sutter wrote:
>>> Jamal,
>>>
>>> On Thu, Feb 04, 2021 at 08:19:55AM -0500, Jamal Hadi Salim wrote:
>>>> I couldnt tell by inspection if what used to work before continues to.
>>>> In particular the kernel version does consider the divisor when folding.
>>>
>>> That's correct. And so does tc. What's the matter?
>>>
>>
>> tc assumes 256 when undefined. Maybe man page needs to be
>> updated to state we need divisor specified otherwise default
>> is 256.
> 
> tc-u32.8 mentions the default in 'sample' option description. Specifying
> divisor is mandatory when creating a hash table, so that path is
> covered, too. I still don't get how this is related to my patch, though.
> 

It is a general comment related to this code (that you are modifying).
You mentioned divisor in your earlier email as part of the syntax for
sample. So it made me wonder:
Does the bucket placement assume a specific number of buckets in a
table? Example if i had a hash table with 4 buckets, would the sample
then pick the correct bucket? Would it be also correct for 32 buckets,
etc. Or it didnt matter before and it doesnt matter now.

>>>> Two examples that currently work, if you can try them:
>>>
>>> Both lack information about the used hashkey and divisor.
>>>
>>>> Most used scheme:
>>>> ---
>>>> tc filter add dev $DEV parent 999:0  protocol ip prio 10 u32 \
>>>> ht 2:: \
>>>> sample ip protocol 1 0xff match ip src 1.2.3.4/32 flowid 1:10 \
>>>> action ok
>>>> ----
>>>
>>> htid before: 0x201000
>>> htid after: 0x201000
>>>
>>
>> Ok, this is the most common use-case. So we are good.
> 
> Whatever.

Meaning?


cheers,
jamal