linux-kernel - Re: [PATCH 7/7] x86,switch_mm: skip atomic operations for init

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Message-ID: <20180715230430.GD30102@gmail.com>
Date:   Mon, 16 Jul 2018 01:04:30 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, luto@...nel.org,
        dave.hansen@...el.com, kernel-team@...com, efault@....de,
        tglx@...utronix.de, songliubraving@...com, hpa@...or.com
Subject: Re: [PATCH 7/7] x86,switch_mm: skip atomic operations for init_mm


* Rik van Riel <riel@...riel.com> wrote:

> +		/*
> +		 * Stop remote flushes for the previous mm.
> +		 * Skip the idle task; we never send init_mm TLB flushing IPIs,
> +		 * but the bitmap manipulation can cause cache line contention.
> +		 */
> +		if (real_prev != &init_mm) {
> +			VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu,
> +						mm_cpumask(real_prev)));
> +			cpumask_clear_cpu(cpu, mm_cpumask(real_prev));

BTW., could this optimization be (safely) extended to all (or most) !task->mm 
kernel threads?

In particular softirq and threaded irq handlers could benefit greatly I suspect in 
certain networking intense workloads that happen to active them.

Thanks,

	Ingo