Date:   Wed, 28 Oct 2020 15:15:58 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Elliot Berman <eberman@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "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 Wed, Oct 28, 2020 at 02:55:16PM +0000, Qais Yousef wrote:
> Hi Elliot
> 
> + Sudeep
> 
> On 10/27/20 10:06, Elliot Berman wrote:
> > 
> > On 10/26/2020 10:12 AM, Peter Zijlstra wrote:
> > > On Mon, Oct 26, 2020 at 10:08:47AM -0700, psodagud@...eaurora.org wrote:
> > > > 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.

Completely agreed, this is what I have suggested people in the past.

> > > >
> > > > 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.
> > >

Either offline "unwanted" CPUs from user space. If that is not possible
for whatever thermal reasons, we need to check if we can disable them in
the DT like ACPI does. IIUC, it is not supported for some reasons I need
to recall/check, can't remember that now. If that is not possible, make
those nodes disappear in the bootloader ?

> > > That's a lot of words, but exactly 0 on _WHY_ you would want to do that.
> > >
> >
> > We find the ability to limit the number of cpus brought online at bootup
> > useful, and to possibly later enable those cores. One use case is when
> > device is undergoing initial testing is to use bootcpus to limit bootup to
> > only a couple cores and later bring up the other cores for a controlled
> > stress test. A core brought up during boot is also running device
> > initialization. Besides being useful for SoC vendor bringup which typically
> > occurs downstream, this particular use case could be exercised by developer
> > of upstream support for a SoC when initial CPU settings are being
> > determined.
> >

Why not try single core instead of couple of core and add the needed ones
for the user-space ?

> > Another use case is if user wishes to limit bootup only to the smaller or
> > bigger cores. maxcpus= is not sufficient here to ensure that only those
> > cores are booted since it limits only to the first N cores, which may not be
> > the desired small or big cores. User may want to bring up only the smaller
> > cores during bootup for thermal reasons. For instance, device may be later
> > sufficiently charged such that boot up of the bigger cores is now
> > permissible. Relying on thermal drivers to later take care of putting core
> > into lower power idle may not occur until much later in boot (for instance,
> > if the governor is a module).
>
> I would have thought that FW/SCP would have the power to block booting up the
> CPUs if it deemed that to be unsafe.
>

I think it is more like *desire* to run with whatever battery life is left
rather than *unsafe* to bring up the core.

Also not sure if we can put such battery life related policies in the
firmware. If there is a thermal constraint, I am sure f/w will and must
refuse to boot the core. I doubt if we are talking about that here. It is
more a policy to extract max out of the battery life left, at-least the way
I see this issue. I may not have full context here, sorry.

-- 
Regards,
Sudeep

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