Re: exercising current aarch64 kprobe support with systemtap

[William Cohen <wcohen at redhat dot com>]

On 06/10/2016 05:28 PM, William Cohen wrote:
> 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
> 

The soft lockup remains even if when the stabk backtrace in systemtap script is disabled: http://paste.stg.fedoraproject.org/5325/

-Will