linux-kernel - Re: [PATCH] x86/apic: reduce cache line misses in __x2apic_send_IPI

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Message-ID: <20211007072917.GN174703@worktop.programming.kicks-ass.net>
Date:   Thu, 7 Oct 2021 09:29:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH] x86/apic: reduce cache line misses in
 __x2apic_send_IPI_mask()

On Wed, Oct 06, 2021 at 08:17:56PM -0700, Eric Dumazet wrote:
> +/* __x2apic_send_IPI_mask() possibly needs to read
> + * x86_cpu_to_logical_apicid for all online cpus in a sequential way.
> + * Using per cpu variable would cost one cache line per cpu.
> + */

Broken comment style..

> +static u32 x86_cpu_to_logical_apicid[NR_CPUS] __read_mostly;

NR_CPUS is really sad, could this at all be dynamically allocated? Say
in x2apic_cluster_probe() ?


Otherwise this looks very nice!