On Sun, 3 Jan 2021 15:30:30 +0900 Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote: > On Sun, Jan 3, 2021 at 5:14 AM Valdis Klētnieks <valdis.kletnieks@xxxxxx> wrote: > > > > 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.
