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

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Date:   Mon, 26 Oct 2020 10:08:47 -0700
From:   psodagud@...eaurora.org
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Elliot Berman <eberman@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Trilok Soni <tsoni@...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 2020-10-23 14:59, Thomas Gleixner wrote:
> 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.

Hi Thomas and Peter,

Based on my understanding with maxcpus option provides, maximum no of 
CPUs are brough up during the device boot up. There is a different case, 
in which we want to restrict which CPUs to be brough up.
On a system with 8 cpus, if we set maxcpus as 3, cpu0, cpu1, and cpu2 
are brough up during the bootup.  For example, if we want to bring 
core0, core3 and core4 current maxcpu(as 3) setting would not help us.
On some platform we want the flexibility on which CPUs to bring up 
during the device bootup. bootcpus command line is helping to bring 
specific CPUs and these patches are working downstream.

> 
> Thanks,
> 
>         tglx