netdev - Re: tc: u32: Wrong sample hash calculation

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Date:   Sun, 24 Jan 2021 08:13:53 -0500
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Phil Sutter <phil@....cc>,
        Stephen Hemminger <stephen@...workplumber.org>,
        netdev@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Russell Stuart <russell-lartc@...art.id.au>
Subject: Re: tc: u32: Wrong sample hash calculation

Hi Phil,

On 2021-01-22 8:59 a.m., Phil Sutter wrote:
> Jamal,
> 
> On Fri, Jan 22, 2021 at 06:25:22AM -0500, Jamal Hadi Salim wrote:
> [...]
>> My gut feel is user space is the right/easier spot to fix this
>> as long as it doesnt break the working setup of 8b.
> 
> One last attempt at clarifying the situation:
> 
> Back in 2004, your commit 4e54c4816bf ("[NET]: Add tc extensions
> infrastructure.")[1] was applied which commented out the old hash
> folding and introduced the shift/cutoff we have today:
> 
> |  @@ -90,10 +101,12 @@ static struct tc_u_common *u32_list;
> |
> |  static __inline__ unsigned u32_hash_fold(u32 key, struct tc_u32_sel *sel)
> |  {
> | -	unsigned h = key & sel->hmask;
> | +	unsigned h = (key & sel->hmask)>>sel->fshift;
> |
> | +	/*
> |  	h ^= h>>16;
> |  	h ^= h>>8;
> | +	*/
> |  	return h;
> |  }
> 
> In a later commit, the new code was made compile-time selected via '#ifdef
> fix_u32_bug'. In that same commit, I don't see a related #define though.
> 
> Do you remember why this was changed? Seems like the old code was
> problematic somehow.
> 

Vague recollection that it didnt work. I will have to dig deeper in old
email exchanges. Its been like close to 20 years (if you consider there
was work in progress about 1-2 years before that final submission) ;->

cheers,
jamal