Re: Reduce Linux boot time on Large scale system

[Peter Zijlstra]

On Tue, Apr 04, 2017 at 04:39:06PM +0000, Noam Camus wrote:
> Hi Peter & Vineet
> 
> I wish to reduce boot time of my platform ARC/plat-eznps (4K CPUs).
> My analysis is that most boot time is spent over cpu_up() for all CPUs
> Measurements are about 66mS per CPU and Totally over 4 minutes (I got 800MHz cores).
> 
> I see that smp_init() just iterate over all present cpus one by one.
> I wish to know if there was an attempt to optimize this with some parallel work?
> 
> Are you aware of some method / trick  that will help me to reduce boot time?
> Any suggestion how this can be done?

So attempts have been made in the past but Thomas shot them down for
being gross hacks (they were).

But Thomas has now (mostly) completed rewriting the CPU hotplug
machinery and he has at some point outlined means of achieving what
you're after.

I've added him to Cc so he can correct me where I'm wrong, as I've not
looked into this in much detail after he mucked up all I knew about CPU
hotplug.

Since each CPU is now responsible for its own bootstrap, we can now kick
all the CPUs awake without waiting for them to complete the online
stage.

There might however be code that assumes CPUs come up one at a time, so
you'll need to audit for that. Its not going to be a trivial thing.