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

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Message-ID: <871rha148x.fsf@nanos.tec.linutronix.de>
Date:   Tue, 03 Nov 2020 23:10:06 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sudeep Holla <sudeep.holla@....com>,
        Elliot Berman <eberman@...eaurora.org>
Cc:     Qais Yousef <qais.yousef@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Sudeep Holla <sudeep.holla@....com>,
        Trilok Soni <tsoni@...eaurora.org>,
        linux-kernel@...r.kernel.org, psodagud@...eaurora.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH] smp: Add bootcpus parameter to boot subset of CPUs

On Fri, Oct 30 2020 at 17:45, Sudeep Holla wrote:
> On Thu, Oct 29, 2020 at 02:37:06PM -0700, Elliot Berman wrote:
>> In the case where commercial device is using feature for thermal, device
>> should boot multiple small cores. Booting only one core means we would not
>> be able to use all possible cores to maximum extent possible in this thermal
>> case.
>
> I understood that point. But you haven't responded on my logical vs physical
> number argument. I am clearly NACKing this patch as is for just usage of
> logical CPU IDs in the command line while your intention is to control
> the physical CPUs. So once again, NACK for that reason.

Correct. And no, we are not going to add a command line option to select
physical CPU ids.

There are two ways to solve that:

  1) The firmware can tell the kernel whether a CPU should be brought up
     or not, e.g. by failing the bootup request.

  2) The kernel has a way to figure out the current thermal and/or power
     budget early in the boot process and sorts out which and how many
     CPUs fit into that limit.

Thanks,

        tglx