[ltt-dev] use percpu variable for ltt_nesting

[Jiaying Zhang]

Hi Mathieu,

I found lttng sometimes has very poor performance on multiple cpu systems
and it seems
the more processors the system has, the more performance overhead I saw with
lttng enabled.
Here are the results I collected with tbench benchmark (
http://samba.org/ftp/tridge/dbench/).

single processor:
   lttng disabled: 236.07 MB/sec
   lttng enabled:  210.569 MB/sec

16 processors:
   lttng disabled:  4173.83 MB/sec
   lttng enabled:  1766.77 MB/sec

After playing with the code for a while and asking around, I found this
performance
issue is caused by the cpu contention while updating the ltt_nesting
variable used
in ltt/ltt-serialize.c:ltt_vtrace:
  ltt_nesting[smp_processor_id()]++;
  ...
  ltt_nesting[smp_processor_id()]--;

In the attachment is the patch that changes ltt_nesting into a per_cpu
variable. With the patch applied,
the tbench performance with lttng applied gets to about 3600 MB/sec on the
16 processor system.

Jiaying
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