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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Wed, 10 Nov 2021 11:24:12 +0100
Message-ID: <CAMj1kXF8makQnZaWDpyzQc2ZiwQEU1ACYhrA91UaFT6S-6RXaQ@mail.gmail.com>
Subject: Re: [PATCH 2/2] jump_label: refine placement of static_keys
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jason Baron <jbaron@akamai.com>,
        "Steven Rostedt (VMware)" <rostedt@goodmis.org>
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On Wed, 10 Nov 2021 at 09:36, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Nov 09, 2021 at 05:09:06PM -0800, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@google.com>
> >
> > With CONFIG_JUMP_LABEL=y, "struct static_key" content is only
> > used for the control path.
> >
> > Marking them __read_mostly is only needed when CONFIG_JUMP_LABEL=n.
> > Otherwise we place them out of the way to increase data locality.
> >
> > This patch adds __static_key to centralize this new policy.
> >
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > ---
> >  arch/x86/kvm/lapic.c       |  4 ++--
> >  arch/x86/kvm/x86.c         |  2 +-
> >  include/linux/jump_label.h | 25 +++++++++++++++++--------
> >  kernel/events/core.c       |  2 +-
> >  kernel/sched/fair.c        |  2 +-
> >  net/core/dev.c             |  8 ++++----
> >  net/netfilter/core.c       |  2 +-
> >  net/netfilter/x_tables.c   |  2 +-
> >  8 files changed, 28 insertions(+), 19 deletions(-)
> >
>
> Hurmph, it's a bit cumbersome to always have to add this __static_key
> attribute to every definition, and in fact you seem to have missed some.
>
> Would something like:
>
>         typedef struct static_key __static_key static_key_t;
>
> work? I forever seem to forget the exact things you can make a typedef
> do :/

No, that doesn't work. Section placement is an attribute of the symbol
not of its type. So we'll need to macro'ify this.

But I'm not sure I understand why we need different policies here.
Static keys are inherently __read_mostly (unless they are not writable
to begin with), so keeping them all together in one place in the
binary should be sufficient, no?