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

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Date:   Thu, 4 Feb 2021 13:08:58 -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 11:50 a.m., Phil Sutter wrote:
> On Thu, Feb 04, 2021 at 10:28:26AM -0500, Jamal Hadi Salim wrote:
>> 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.
> 
> My patch doesn't change how divisor is applied. And yes, with a smaller
> than 256 buckets hash table, specifying the divisor along with sample is
> necessary.
> 

Sorry - hadnt looked at the man page earlier. I think the text added is
sufficient. I am wondering why we made the divisor optional to begin
with.

cheers,
jamal