Re: exercising current aarch64 kprobe support with systemtap

[William Cohen <wcohen@redhat.com>]

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On 06/13/2016 12:27 AM, Pratyush Anand wrote:
> Hi Will,
> 
> On 10/06/2016:05:28:36 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
> 
> Now we have following in arch_within_kprobe_blacklist(). So above issue should
> not bite us.
> 
> +           !!search_exception_tables(addr))
> +               return true;
> 
>>
>> 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?
> 
> Not sure, could be the locked up CPU waiting for a lock (spinlock), which is not
> being released. Just noticed that, backtrace of all active CPUs (`echo l >
> /proc/sysrq-trigger`) is not working for arm64. Probably because, we do not have
> arch_trigger_all_cpu_backtrace() defined for aarch64. May be we can have one,
> like that of arm. Backtrace of CPUs in this state might give us some input.
> 
> ~Pratyush
> 

Hi Pratyush,

I did some additional experimentation this weekend.  The version of systemtap script with an empty body (the attached hw_watch_addr_null2.stp) still caused the system to have soft lockup.  However, the equivalent perf use of the hardware watchpoint worked fine (it got counts and no soft lookup):

 perf stat -a -e mem:0x`grep "vm_dirty_ratio" /proc/kallsyms | awk '{print $1}'`/1 bash

So it looks like the issue might lie with something in systemtap.

-Will

[-- Attachment #2: hw_watch_addr_null2.stp --]
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#! /usr/bin/env stap

%( CONFIG_HAVE_HW_BREAKPOINT == "y" %?
probe kernel.data($1).rw
{
}
%:
probe never {}
%)