linux-kernel - Re: [PATCH v6 1/9] smp: Run functions concurrently in smp_call_function_many

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Date:   Mon, 1 Mar 2021 18:10:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nadav Amit <namit@...are.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...riel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v6 1/9] smp: Run functions concurrently in
 smp_call_function_many_cond()

On Sat, Feb 20, 2021 at 03:17:04PM -0800, Nadav Amit wrote:
> +		/*
> +		 * Choose the most efficient way to send an IPI. Note that the
> +		 * number of CPUs might be zero due to concurrent changes to the
> +		 * provided mask.
> +		 */
> +		if (nr_cpus == 1)
> +			arch_send_call_function_single_ipi(last_cpu);
> +		else if (likely(nr_cpus > 1))
> +			arch_send_call_function_ipi_mask(cfd->cpumask_ipi);

I just ran into conflicts with another patch set, and noticed that the
above should probably be:

		if (nr_cpus == 1)
			send_call_function_single_ipi(last_cpu);
		else if (likely(nr_cpus > 1))
			arch_send_call_function_ipi_mask(cfd->cpumask_ipi);

Which will avoid the IPI when @last_cpu is idle.