[patch] ARM: smpboot: Enable interrupts after marking CPU online/active

[Thomas Gleixner]

On Tue, 13 Sep 2011, Russell King - ARM Linux wrote:

> On Tue, Sep 13, 2011 at 07:22:16PM +0200, Vincent Guittot wrote:
> > The assumption done in the 1st patch that smp_store_cpu_info can be
> > delayed is no more true. The smp_store_cpu_info is now also used to
> > store the cpu topology information
> > (https://lkml.org/lkml/2011/7/5/209). This must be done before calling
> > sched_init_smp, which will use this information for building
> > sched_domain.
> > If we move set_cpu_online before smp_store_cpu_info, sched_init_smp
> > can be called before having called smp_store_cpu_info on all cpus.
> 
> Right.  We hold off returning from cpu_up() by watching for the upcoming
> CPU setting its online bit.
> 
> The bug which Thomas' patch introduces is to move the setting of that
> before we've finished bringing the CPU up - specifically, allowing the
> requesting CPU to continue while the brought-up CPU is still calibrating
> loops_per_jiffy, and before it's stored that information and setup the
> scheduler domain information.
> 
> The other issue is that moving the marking of the CPU online in the
> way Thomas has means that we then invite the delivery of IPIs to the
> CPU which is still in the process of coming up.  Whether that's an
> issue depends on what the IPIs are.
> 
> So, we must have the setting of CPU online _after_ we've setup the
> scheduler domain information etc - so the following is a strict
> ordering:
> 
> 1. calibrate_delay()
> 2. smp_store_cpu_info()
> 3. set_cpu_online()
> 
> Now, the question is do we need interrupts enabled to setup timers
> via percpu_timer_setup() and calibrate delay.  Can we move enabling
> interrupts after smp_store_cpu_info().  IOW, instead of moving the
> setting of cpu online before all this, can we move notify_cpu_starting()
> and the enabling of _both_ interrupts after smp_store_cpu_info()...
> No idea at the moment.

The question is whether you really need to calibrate the delay for
each core or if you simply can take the calibration of the first
core. That would avoid enabling interrupts all the way.

Thanks,

	tglx