Re: exercising current aarch64 kprobe support with systemtap

[William Cohen <wcohen@redhat.com>]

On 06/09/2016 12:17 PM, William Cohen wrote:
> I have been exercising the current kprobes and uprobe patches for
> arm64 that are in the test_upstream_arm64_devel branch of
> https://github.com/pratyushanand/linux with systemtap.  There are a
> two issues that I have seen on this kernel with systemtap.  There are
> some cases where kprobes fail to register at places that appear to be
> reasonable places for a kprobe.  The other issue is that kernel starts
> having soft lockups when the hw_watch_addr.stp tests runs.  To get
> systemtap with the newer kernels need the attached hack because of
> changes in the aarch64 macro args.
...
> Soft Lookup for the hw_watch_addr.stp
> 
> When running the hw_watch_addr.stp tests the machine gets a number of
> processes using a lot of sys time and eventually the kernel reports
> soft lockup:
> 
> http://paste.stg.fedoraproject.org/5323/
> 
> The systemtap.base/overload.exp tests all pass, but maybe there is
> much work being done to generate the backtraces for hw_watch_addr.stp
> and that is triggering the problem.

I can reliably reproduce the soft lockup running a single test with:

/root/systemtap_write/install/bin/stap --all-modules \
/root/systemtap_write/systemtap/testsuite/systemtap.examples/memory/hw_watch_addr.stp \
0x`grep "vm_dirty_ratio" /proc/kallsyms | awk '{print $1}'` -T 5 > /dev/null

paste of output and soft lockup at: http://paste.stg.fedoraproject.org/5324/

One of the things that Jeremy Linton pointed to was:

https://lkml.org/lkml/2016/3/21/198

Could the aarch64 hardware watchpoint handler have an issue that is causing this problem with the soft lockup?
Or spending too much time doing the stack backtrace?

-Will