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Re: [tracecompass-dev] Kernel Lock Analysis

Wow, this is really nice, would you be interested in contributing this
to the incubator, it looks like a prime candidate for the "LTTng Extras".

On 18-04-10 12:38 PM, aleix wrote:
> Hello List!
> Just as I have done with the LTTng people I would like to share my
> experience playing with Kernel lock instrumentation, LTTng and Trace
> Compass which might be useful for others when analyzing applications
> behavior.
> Currently, the Linux kernel features a set of lock tracepoints which
> track lock acquisition and contention [1]:
>  - lock_acquire
>  - lock_acquired
>  - lock_contended
>  - lock_release
> However, the current version of LTTng does not instrument this
> tracepoints (the code is in the source code but commented [2]). The
> guys on LTTng IRC helped me enabling them again by just uncommenting
> the code. I have currently reported a bug about this here [3].
> Thanks to these tracepoints, I could create a couple of Trace Compass
> views (attached in this mail), to easily track the kernel lock
> contention, which clearly showed me what was going on. One view shows
> locking status per each kernel lock and the other shows per thread
> contention for all locks). Please, see the attached screenshot for an
> example of per thread view.
> The application under analysis is a parallel cholesky benchmark run on
> a server with 56 CPUs. I was trying to figure out why almost all
> application threads became blocked at some point as seen in the
> screenshot. The lock view showed that there was a huge contention on
> the mm->mmap_sem lock when all threads tried to allocate memory by
> calling mmap(), mmprotect() and triggered page faults when data is
> written on the recently mmapped memory.
> Hence, I would like to point out how useful it has been for me to
> enable the LTTng lock tracepoints and use the custom lock views. If
> the lock tracepoints are enabled again, I think that it might useful
> for others to add similar views. However, It is worth noting that this
> tracepoints are only enabled if compiling the kernel with
> CONFIG_LOCK_STAT, which is a kernel debugging feature not set by
> default.
> Thanks for your work!
>  [1] See Documentation/locking/lockstat.txt on the Linux Kernel source
>      for more information.
>  [2] As Compudj pointed out, the reason is found in this conversation:
>  [3] LTTng lock bug:
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