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

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Date:   Fri, 2 Sep 2022 20:02:46 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 22/59] x86: Put hot per CPU variables into a struct

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