mm/zsmalloc: add trace events for zs_compact

mm/zsmalloc: add trace events for zs_compact

Currently zsmalloc is widely used in android device.  Sometimes, we want
to see how frequently zs_compact is triggered or how may pages freed by
zs_compact(), or which zsmalloc pool is compacted.

We have backported the zs_compact() to our product(kernel 3.18).  It is
usefull for a longtime running device.  But there is not a convenient
way to get the detailed information of zs_comapct() which is usefull
for performance optimization.  Information about how much time
zs_compact used, which pool is compacted, how many page freed, etc.
With these information, we will know what is going on in zs_comapct.
And draw the relation between free mem and zs_comapct.

Most of the time, user can get the brief information from
trace_mm_shrink_slab_[start | end], but in some senario, they do not use
zsmalloc shrinker, but trigger compaction manually.  So add some trace
events in zs_compact is convenient.  Also we can add some zsmalloc
specific information(pool name, total compact pages, etc) in zsmalloc
trace.

This patch add two trace events for zs_compact(), below the trace log:
-----------------------------
root@land:/ # cat /d/tracing/trace
         kswapd0-125   [007] ...1   174.176979: zsmalloc_compact_start: pool zram0
         kswapd0-125   [007] ...1   174.181967: zsmalloc_compact_end: pool zram0: 608 pages compacted(total 1794)
         kswapd0-125   [000] ...1   184.134475: zsmalloc_compact_start: pool zram0
         kswapd0-125   [000] ...1   184.135010: zsmalloc_compact_end: pool zram0: 62 pages compacted(total 1856)
         kswapd0-125   [003] ...1   226.927221: zsmalloc_compact_start: pool zram0
         kswapd0-125   [003] ...1   226.928575: zsmalloc_compact_end: pool zram0: 250 pages compacted(total 2106)
-----------------------------

Link: http://lkml.kernel.org/r/1465289804-4913-1-git-send-email-opensource.ganesh@gmail.com
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>