Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling

[Steven Rostedt]

On Wed, 21 Feb 2024 11:41:47 -0800
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:

> > I wonder if we can just see if the instruction pointer at preemption is at
> > something that was allocated? That is, if it __is_kernel(addr) returns
> > false, then we need to do more work. Of course that means modules will also
> > trigger this. We could check __is_module_text() but that does a bit more
> > work and may cause too much overhead. But who knows, if the module check is
> > only done if the __is_kernel() check fails, maybe it's not that bad.  
> 
> I do like very much that idea, but it requires that we be able to identify
> this instruction pointer perfectly, no matter what.  It might also require
> that we be able to perfectly identify any IRQ return addresses as well,
> for example, if the preemption was triggered within an interrupt handler.
> And interrupts from softirq environments might require identifying an
> additional level of IRQ return address.  The original IRQ might have
> interrupted a trampoline, and then after transitioning into softirq,
> another IRQ might also interrupt a trampoline, and this last IRQ handler
> might have instigated a preemption.

Note, softirqs still require a real interrupt to happen in order to preempt
executing code. Otherwise it should never be running from a trampoline.

> 
> Are there additional levels or mechanisms requiring identifying
> return addresses?

Hmm, could we add to irq_enter_rcu()

	__this_cpu_write(__rcu_ip, instruction_pointer(get_irq_regs()));

That is to save off were the ip was when it was interrupted.

Hmm, but it looks like the get_irq_regs() is set up outside of
irq_enter_rcu() :-(

I wonder how hard it would be to change all the architectures to pass in
pt_regs to irq_enter_rcu()? All the places it is called, the regs should be
available.

Either way, it looks like it will be a bit of work around the trampoline or
around RCU to get this efficiently done.

-- Steve