Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220902130625.217071627@infradead.org> <20220902130948.643735860@infradead.org>
In-Reply-To: <20220902130948.643735860@infradead.org>
From:   Jann Horn <jannh@google.com>
Date:   Fri, 2 Sep 2022 20:02:46 +0200
Message-ID: <CAG48ez07c2ThydovaHtBZ+u137eaXdEut4J-zycvPn7io4MySw@mail.gmail.com>
Subject: Re: [PATCH v2 22/59] x86: Put hot per CPU variables into a struct
To:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Johannes Wikner <kwikner@ethz.ch>,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        "H.J. Lu" <hjl.tools@gmail.com>,
        Joao Moreira <joao.moreira@intel.com>,
        Joseph Nuzman <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Juergen Gross <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        Eric Dumazet <edumazet@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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.