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

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Message-ID: <20210204165020.GU3158@orbyte.nwl.cc>
Date:   Thu, 4 Feb 2021 17:50:20 +0100
From:   Phil Sutter <phil@....cc>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        netdev@...r.kernel.org
Subject: Re: [iproute PATCH] tc: u32: Fix key folding in sample option

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.

> >>>> 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?

Things are always fine if we only consider the positive results.

Cheers, Phil