linux-kernel - Re: [PATCH] smp: Add bootcpus parameter to boot subset of CPUs

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Message-ID: <87v9f04n8r.fsf@nanos.tec.linutronix.de>
Date:   Fri, 23 Oct 2020 23:59:48 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Elliot Berman <eberman@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Jonathan Corbet <corbet@....net>
Cc:     Elliot Berman <eberman@...eaurora.org>,
        Trilok Soni <tsoni@...eaurora.org>,
        Prasad Sodagudi <psodagud@...eaurora.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH] smp: Add bootcpus parameter to boot subset of CPUs

On Thu, Oct 22 2020 at 15:04, Elliot Berman wrote:
> In a heterogeneous multiprocessor system, specifying the 'maxcpus'
> parameter on kernel command line does not provide sufficient control
> over which CPUs are brought online at kernel boot time, since CPUs may
> have nonuniform performance characteristics. Thus, add bootcpus kernel
> parameter to control which CPUs should be brought online during kernel
> boot. When both maxcpus and bootcpus is set, the more restrictive of the
> two are booted.

What for? 'maxcpus' is a debug hack at best and outright dangerous on
certain architectures. Why do we need more of that? Just let the machine
boot and offline the CPUs from user space.

Thanks,

        tglx