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Date:   Tue, 25 Jun 2019 03:11:18 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Nicholas Piggin <npiggin@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Qais Yousef <qais.yousef@arm.com>
Subject: Re: [PATCH v2] kernel/isolation: Assert that a housekeeping CPU
 comes up at boot time
Message-ID: <20190625011117.GC17497@lerouge>
References: <20190625001720.19439-1-npiggin@gmail.com>
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On Tue, Jun 25, 2019 at 10:17:20AM +1000, Nicholas Piggin wrote:
> +static int __init housekeeping_verify_smp(void)
> +{
> +	int cpu;
> +
> +	if (!housekeeping_flags)
> +		return 0;
> +
> +	/*
> +	 * Early housekeeping setup is done before CPUs come up, and there are
> +	 * a range of options scattered around that can restrict which CPUs
> +	 * come up. It is possible to pass in a combination of housekeeping
> +	 * and SMP arguments that result in housekeeping assigned to an
> +	 * offline CPU.
> +	 *
> +	 * Check that condition here after SMP comes up, and give a useful
> +	 * error message rather than an obscure non deterministic crash or
> +	 * hang later.
> +	 */
> +	for_each_online_cpu(cpu) {
> +		if (cpumask_test_cpu(cpu, housekeeping_mask))
> +			return 0;
> +	}
> +	panic("Housekeeping: nohz_full= or isolcpus= resulted in no online CPUs for housekeeping.\n");

Ok let's keep the panic. But let's simplify and spare long iterations off boot load:

    if (!cpumask_intersects(cpu_online_mask, housekeeping_mask))
        panic(...);

Thanks.