Re: [PATCH v2 22/59] x86: Put hot per CPU variables into a struct

[Jann Horn]

On Fri, Sep 2, 2022 at 3:54 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>
> 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.