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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Sudeep Holla <sudeep.holla@arm.com>,
        Elliot Berman <eberman@codeaurora.org>
Cc:     Qais Yousef <qais.yousef@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Trilok Soni <tsoni@codeaurora.org>,
        linux-kernel@vger.kernel.org, psodagud@codeaurora.org,
        linux-doc@vger.kernel.org
Subject: Re: [PATCH] smp: Add bootcpus parameter to boot subset of CPUs
In-Reply-To: <20201030174531.ywwkcntq4ge33lrh@bogus>
References: <1603404243-5536-1-git-send-email-eberman@codeaurora.org> <87v9f04n8r.fsf@nanos.tec.linutronix.de> <a6d7f84679240fcf580520230a88c058@codeaurora.org> <20201026171224.GV2611@hirez.programming.kicks-ass.net> <a9fa1f8d-52c7-adca-9087-160b1ecda6b8@codeaurora.org> <20201028145516.23lm66mora5b3wqr@e107158-lin> <20201028151558.odxwolnwbes2gihi@bogus> <cb175d84-7a89-344d-0dd8-76bf4ece9e3b@codeaurora.org> <20201030174531.ywwkcntq4ge33lrh@bogus>
Date:   Tue, 03 Nov 2020 23:10:06 +0100
Message-ID: <871rha148x.fsf@nanos.tec.linutronix.de>
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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