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Date:   Thu, 15 Sep 2022 13:22:17 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jann Horn <jannh@google.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Linus Torvalds <torvalds@linux-foundation.org>,
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        Daniel Borkmann <daniel@iogearbox.net>,
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        Eric Dumazet <edumazet@google.com>
Subject: Re: [PATCH v2 22/59] x86: Put hot per CPU variables into a struct
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On Fri, Sep 02, 2022 at 08:02:46PM +0200, Jann Horn wrote:
> On Fri, Sep 2, 2022 at 3:54 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > From: Thomas Gleixner <tglx@linutronix.de>
> >
> > The layout of per-cpu variables is at the mercy of the compiler. This
> > can lead to random performance fluctuations from build to build.
> >
> > Create a structure to hold some of the hottest per-cpu variables,
> > starting with current_task.
> [...]
> > -DECLARE_PER_CPU(struct task_struct *, current_task);
> > +struct pcpu_hot {
> > +       union {
> > +               struct {
> > +                       struct task_struct      *current_task;
> > +               };
> > +               u8      pad[64];
> > +       };
> > +};
> 
> fixed_percpu_data::stack_canary is probably also a fairly hot per-cpu
> variable on distro kernels with CONFIG_STACKPROTECTOR_STRONG (which
> e.g. Debian enables), so perhaps it'd make sense to reuse
> fixed_percpu_data as the struct for hot percpu variables? But I don't
> have any numbers to actually back up that idea.

Not a bad idea; but the immediate problem I see with this is that
fixed_percpu_data is x86_64 only.

Also; I'm thinking the current stack-protector thing is somewhat of a
hack due to GCC limitations (per the comment there) and once that gets
cleaned up it can come live in the pcpu_hot thing.