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Date:   Mon, 27 May 2019 11:15:34 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nadav Amit <namit@vmware.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>, linux-kernel@vger.kernel.org,
        Rik van Riel <riel@surriel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [RFC PATCH 3/6] smp: Run functions concurrently in
 smp_call_function_many()
Message-ID: <20190527091534.GS2623@hirez.programming.kicks-ass.net>
References: <20190525082203.6531-1-namit@vmware.com>
 <20190525082203.6531-4-namit@vmware.com>
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> +		/*
> +		 * 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 or cpu_online_mask.
> +		 */

Since we have preemption disabled here, I don't think online mask can
shrink, cpu-offline uses stop_machine().

> +		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);
> +	}