linux-kernel - Re: [PATCH] smp: force all cpu to boot once under maxcpus option

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Message-ID: <alpine.DEB.2.21.1907221137090.1782@nanos.tec.linutronix.de>
Date:   Mon, 22 Jul 2019 11:41:29 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Pingfan Liu <kernelfans@...il.com>
cc:     x86@...nel.org, "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Jiri Kosina <jkosina@...e.cz>,
        Mukesh Ojha <mojha@...eaurora.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] smp: force all cpu to boot once under maxcpus option

On Wed, 10 Jul 2019, Pingfan Liu wrote:
>  
> +static inline bool maxcpus_allowed(unsigned int cpu)
> +{
> +	/* maxcpus only takes effect during system bootup */
> +	if (smp_boot_done)
> +		return true;
> +	if (num_online_cpus() < setup_max_cpus)
> +		return true;
> +	/*
> +	 * maxcpus should allow cpu to set CR4.MCE asap, otherwise the set may
> +	 * be deferred indefinitely.
> +	 */
> +	if (!per_cpu(cpuhp_state, cpu).booted_once)
> +		return true;

As this is a x86 only issue, you cannot inflict this magic on every
architecture.

Aside of that this does not solve the problem at all because smp_init()
still does:

        for_each_present_cpu(cpu) {
                if (num_online_cpus() >= setup_max_cpus)
                        break;
                if (!cpu_online(cpu))
                        cpu_up(cpu);
        }

So the remaining CPUs are not onlined at all.

Thanks,

	tglx