On Mon, Jul 28, 2025 at 05:53:34PM +0200, Nam Cao wrote:
> On Mon, Jul 28, 2025 at 03:50:19PM +0200, Gabriele Monaco wrote:
> > The tss monitor currently guarantees task switches can happen only while
> > scheduling, whereas the sncid monitor enforces scheduling occurs with
> > interrupt disabled.
> >
> > Replace the monitors with a more comprehensive specification which
> > implies both but also ensures that:
> > * each scheduler call disable interrupts to switch
> > * each task switch happens with interrupts disabled
> >
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Gabriele Monaco <gmonaco@xxxxxxxxxx>
>
> I gave this a try on riscv64 and observed some errors:
>
> [ 620.696055] rv: monitor sts does not allow event sched_switch on state enable_to_exit
> [ 621.047705] rv: monitor sts does not allow event sched_switch on state enable_to_exit
> [ 642.440209] rv: monitor sts does not allow event sched_switch on state enable_to_exit
>
> I tested with two user programs:
>
> int main() { asm ("unimp"); }
> int main() { asm ("ebreak"); }
>
> The two programs are repeatedly executed:
>
> #!/bin/bash
> ./test1 &
> ./test2 &
> # ... repeat lots of time
Okay, I think I know why..
It seems the monitor is in scheduling state. Then it sees a pair of
irq_disable and irq_enable, and it mistakenly thinks that this is the
is_switch==false case in __schedule. So it thinks it is at the end of
__schedule(), and does not expect a switch_switch.
However, this is wrong. The irq_disable and irq_enable pair is not from
__schedule(), it is from softirq (see below).
In short, the monitor thinks it is at the end of __schedule(), but actually
it is still at the beginning.
That's just from my limited understanding of the model, so I may be wrong.
What do you think?
Nam
test-256 [002] dns.. 63.070743: da_event_sts: scheduling x irq_disable -> disable_to_switch
test-256 [002] dns.. 63.070748: <stack trace>
=> trace_dump_stack
=> da_event_sts
=> handle_irq_disable
=> trace_hardirqs_off.part.0
=> trace_hardirqs_off
=> note_gp_changes
=> rcu_core
=> rcu_core_si
=> handle_softirqs
=> __irq_exit_rcu
=> irq_exit_rcu
=> handle_riscv_irq
=> call_on_irq_stack
test-256 [002] dns.. 63.070755: da_event_sts: disable_to_switch x irq_enable -> enable_to_exit
test-256 [002] dns.. 63.070760: <stack trace>
=> trace_dump_stack
=> da_event_sts
=> handle_irq_enable
=> trace_hardirqs_on
=> note_gp_changes
=> rcu_core
=> rcu_core_si
=> handle_softirqs
=> __irq_exit_rcu
=> irq_exit_rcu
=> handle_riscv_irq
=> call_on_irq_stack
=> call_on_irq_stack
