On Wed, May 16, 2018 at 01:45:45PM +0000, Ehson Hussain wrote:
> Hello Paul,
>
> Hope you're doing great. I'm trying JTAG debugging on an aarch64 SoC (Xilinx
> Zynqmp zcu102 to be precise); when I halt the cores manually, or hit a
> breakpoint, I get a CPU stall warning on resuming after the RCU grace period
> expires.
>
> If I resume quickly enough, I do not get the warning.
>
> My question is, if all cores are stopped during debugging, how does RCU code
> detect/self-detect stalls on CPU? I have never observed this behavior in
> older kernels (3.14 and above), with arm cores e.g. armv7 based Freescale
> iMx6 boards.
My guess is that the jiffies clock is updated after resume to allow for
the time elapsed during the halt. So the system resumes, the jiffies
counter gets a huge increment, and RCU complains about the massive
passage of time.
If you are doing JTAG debugging, I recommend shutting off RCU CPU stall
warnings, for example, using the rcupdate.rcu_cpu_stall_suppress kernel
boot parameter.
I suppose you could also tweak the clock code to pretend that no time
passes while halted, but that could be a very tricky task.
Thanx, Paul
