Messages in this thread | | | Date | Wed, 21 Feb 2024 15:11:57 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling |
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On Wed, 21 Feb 2024 11:41:47 -0800 "Paul E. McKenney" <paulmck@kernel.org> 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
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