Re: [dpdk-dev] Overriding rte_config.h

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> > > On Fri, Oct 29, 2021 at 09:48:30AM -0400, Ben Magistro wrote:
> > > > With the transition to meson, what is the best way to provide custom values
> > > > to parameters in rte_config.h?  When using makefiles, (from memory, I
> > > > think) we used common_base as a template that was copied in as a
> > > > replacement for defconfig_x86....  Our current thinking is to apply a
> > > > locally maintained patch so that we can track custom values easier to the
> > > > rte_config.h file unless there is another way to pass in an overridden
> > > > value.  As an example, one of the values we are customizing is
> > > > IP_FRAG_MAX_FRAG.
> > > >
> > > > Cheers,
> > > >
> > > There is no one defined way for overriding values in rte_config with the
> > > meson build system, as values there are ones that should rarely need to be
> > > overridden. If it's the case that one does need tuning, we generally want
> > > to look to either change the default so it works for everyone, or
> > > alternatively look to replace it with a runtime option.
> > >
> > > In the absense of that, a locally maintained patch may be reasonable. To
> > > what value do you want to change MAX_FRAG? Would it be worth considering as
> > > a newer default value in DPDK itself, since the current default is fairly
> > > low?
> >
> > That might be an option, with IP_FRAG_MAX_FRAG==8 it should be able
> > to cover common jumbo frame size (9K) pretty easily.
> > As a drawback default reassembly table size will double.
> 
> Maybe not. I'm not an expert in the library, but it seems the basic struct
> used for tracking the packets and fragments is "struct ip_frag_pkt". Due to
> the other data in the struct and the linked-list overheads, the actual size
> increase when doubling MAX_FRAG from 4 to 8 is only 25%. According to gdb
> on my debug build it goes from 192B to 256B.

Ah yes, you right, struct ip_frag should fit into 16B, key seems the biggest one. 

> 
> > Even better would be to go a step further and rework lib/ip_frag
> > to make it configurable runtime parameter.
> >
> Agree. However, that's not as quick a fix as just increasing the default
> max segs value which could be done immediately if there is consensus on it.

You mean for 21.11?
I don't mind in principle, but would like to know other people thoughts here.
Another thing -  we didn't announce it in advance, and it is definitely an ABI change.