On Tue, Jul 29, 2025 at 10:46:51AM +0200, Gabriele Monaco wrote:
> On Mon, 2025-07-28 at 17:53 +0200, Nam Cao wrote:
> > 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
> >
> > Any idea?
>
> Mmh I see what you're doing here..
> Those instructions are supposed to raise some sort of exception in the
> CPU which apparently disables and enables interrupts without raising an
> interrupt handler tracepoint (the discriminator for this monitor).
> This lets the monitor believe we passed the time a switch is possible
> and complain when it actually sees one.
>
> I still couldn't reproduce it on my VM, yet I find the timing a bit
> strange: it's alright we handle the illegal instruction like this, but
> do we really end up doing that while scheduling although it doesn't
> look like an interrupt?!
>
> Could you share a bit more about your riscv setup? It might some
> configuration/hardware specific thing.
Kernel:
- base: ftrace/for-next
- config: defconfig + mod2noconfig + PREEMPT_RT + monitors
Hardware:
qemu-system-riscv64 -machine virt \
-kernel ../linux/arch/riscv/boot/Image \
-append "console=ttyS0 root=/dev/vda rw" \
-nographic \
-drive if=virtio,format=raw,file=riscv64.img \
-smp 4 -m 4G
riscv64.img is a Debian trixie image from debootstrap
Test:
echo 0 > /proc/sys/debug/exception-trace
./testall # see attached
test.sh
Description: Bourne shell script
#!/bin/bash ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh & ./test.sh &
