Re: Kconfig, DEFAULT_NETSCH, and shooting yourself in the foot..

On Sun, 3 Jan 2021 15:30:30 +0900
Masahiro Yamada [off-list ref] wrote:

On Sun, Jan 3, 2021 at 5:14 AM Valdis Klētnieks [off-list ref] wrote:

quoted

Consider the following own goal I just discovered I scored:

[~] zgrep -i fq_codel /proc/config.gz
CONFIG_NET_SCH_FQ_CODEL=m
CONFIG_DEFAULT_FQ_CODEL=y
CONFIG_DEFAULT_NET_SCH="fq_codel"

Obviously, fq_codel didn't get set as the default, because that happens
before the module gets loaded (which may never happen if the sysadmin
thinks the DEFAULT_NET_SCH already made it happen)

Whoops. My bad, probably - but....

The deeper question, part 1:

There's this chunk in net/sched/Kconfig:

config DEFAULT_NET_SCH
        string
        default "pfifo_fast" if DEFAULT_PFIFO_FAST
        default "fq" if DEFAULT_FQ
        default "fq_codel" if DEFAULT_FQ_CODEL
        default "fq_pie" if DEFAULT_FQ_PIE
        default "sfq" if DEFAULT_SFQ
        default "pfifo_fast"
endif

(And a similar chunk right above it with a similar issue)

Should those be "if (foo=y)" so =m can't be chosen? (I'll be
happy to write the patch if that's what we want)

Deeper question, part 2:

Should there be a way in the Kconfig language to ensure that
these two chunks can't accidentally get out of sync?  There's other
places in the kernel where similar issues arise - a few days ago I was
chasing a CPU governor issue where it looked like it was possible
to set a default that was a module and thus possibly not actually loaded.
 

If there is a restriction where a modular discipline cannot be the default,
I think you can add 'depends on FOO = y'.



For example,


choice
           prompt "Default"

           config DEFAULT_FOO
                  bool "Use foo for default"
                  depends on FOO = y

           config DEFAULT_BAR
                  bool "Use bar for default"
                  depends on BAR = y

           config DEFAULT_FALLBACK
                  bool "fallback when nothing else is builtin"

endchoice

You can use a qdisc that is a module, it just has to be available when device
is loaded. Typically that means putting it in initramfs.