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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 7 Sep 2021 16:49:35 -0700
Message-ID: <CAHk-=wiynwuneR4EbUNtd2_yNT_DR0VQhUF1QOZ352D-NOncjQ@mail.gmail.com>
Subject: Re: ipv4/tcp.c:4234:1: error: the frame size of 1152 bytes is larger
 than 1024 bytes [-Werror=frame-larger-than=]
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Naresh Kamboju <naresh.kamboju@linaro.org>,
        Mathias Nyman <mathias.nyman@intel.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Brendan Higgins <brendanhiggins@google.com>,
        Ariel Elior <aelior@marvell.com>,
        GR-everest-linux-l2@marvell.com, Wei Liu <wei.liu@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        open list <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>, lkft-triage@lists.linaro.org,
        Arnd Bergmann <arnd@arndb.de>,
        "David S. Miller" <davem@davemloft.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Eric Dumazet <edumazet@google.com>
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On Tue, Sep 7, 2021 at 4:35 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Won't your example only fix the issue with CONFIG_CPUMASK_OFFSTACK=y

Yes, but..

> or am I misreading the gigantic comment in include/linux/cpumask.h?

you're not misreading the comment, but you are missing this important fact:

  config NR_CPUS_RANGE_END
        int
        depends on X86_64
        default 8192 if  SMP && CPUMASK_OFFSTACK
        default  512 if  SMP && !CPUMASK_OFFSTACK
        default    1 if !SMP

so basically you can't choose more than 512 CPU's unless
CPUMASK_OFFSTACK is set.

Of course, we may have some bug in the Kconfig elsewhere, and I didn't
check other architectures. So maybe there's some way to work around
it.

But basically the rule is that CPUMASK_OFFSTACK and NR_CPUS are linked.

That linkage is admittedly a bit hidden and much too subtle. I think
the only real reason why it's done that way is because people wanted
to do test builds with CPUMASK_OFFSTACK even without having to have
some ludicrous number of NR_CPUS.

You'll notice that the question "CPUMASK_OFFSTACK" is only enabled if
DEBUG_PER_CPU_MAPS is true.

That whole "for debugging" reason made more sense a decade ago when
this was all new and fancy.

It might make more sense to do that very explicitly, and make
CPUMASK_OFFSTACK be just something like

  config NR_CPUS_RANGE_END
        def_bool NR_CPUS <= 512

and get rid of the subtlety and choice in the matter.

             Linus