Just like we do for userspace when reading the symtab, reducing the
number of entries we insert on the symbols rbtree.

Before:

[acme@emilia ~]$ rm -f perf_report.perf ; perf record -o perf_report.perf perf stat perf report > /dev/null

 Performance counter stats for 'perf':

     218.138382  task clock ticks     (msecs)
              4  context switches     (events)
              8  CPU migrations       (events)
           2136  pagefaults           (events)
       32746212  CPU cycles           (events)  (scaled from 67.04%)
       11961102  instructions         (events)  (scaled from 66.19%)
          49841  cache references     (events)  (scaled from 21.96%)
          13777  cache misses         (events)  (scaled from 21.98%)

 Wall-clock time elapsed:   218.702477 msecs

[acme@emilia ~]$ perf report -i perf_report.perf | head
11.06          perf [.] 0x00000000000057cb /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: dso__find_symbol
 9.15          perf [.] 0x00000000000056a0 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: dso__insert_symbol
 8.72          perf [k] 0xffffffff8101b1d2 intel_pmu_enable_all
 8.51          perf [.] 0x0000000000006672 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: thread__symbol_incnew
 3.83          perf [k] 0xffffffff811cfc5a vsnprintf
 3.40          perf [.] 0x0000000000005e33 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: hex
 3.40          perf [.] 0x0000000000005ec7 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: hex2long
 3.19          perf [k] 0xffffffff811ce1c1 number
 2.77          perf [.] 0x0000000000006869 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: threads__findnew
 2.77          perf [.] 0x000000000000fde3 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: rb_insert_color
[acme@emilia ~]$

After:

acme@emilia ~]$ rm -f perf_report.perf ; perf record -o perf_report.perf perf stat perf report > /dev/null

 Performance counter stats for 'perf':

     190.228511  task clock ticks     (msecs)
              4  context switches     (events)
              7  CPU migrations       (events)
           1625  pagefaults           (events)
       29578745  CPU cycles           (events)  (scaled from 66.92%)
       10516914  instructions         (events)  (scaled from 66.47%)
          44015  cache references     (events)  (scaled from 22.04%)
           8248  cache misses         (events)  (scaled from 22.07%)

 Wall-clock time elapsed:   190.816096 msecs

[acme@emilia ~]$ perf report -i perf_report.perf | head
15.99          perf [.] 0x00000000000057a9 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: dso__find_symbol
10.87          perf [.] 0x000000000000674d /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: thread__symbol_incnew
 8.74          perf [k] 0xffffffff8101b1d2 intel_pmu_enable_all
 5.54          perf [.] 0x0000000000005e42 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: hex
 4.48          perf [.] 0x0000000000005ebe /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: hex2long
 4.48          perf [k] 0xffffffff811cfba0 vsnprintf
 3.84          perf [.] 0x00000000000056b4 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: dso__insert_symbol
 3.62          perf [.] 0x00000000000068d0 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: threads__findnew
 3.20          perf [k] 0xffffffff811ce0b3 number
 2.56          perf [.] 0x0000000000006d78 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: __cmd_report
[acme@emilia ~]$

[ Impact: optimization ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <20090526222155.GJ4424@ghostprotocols.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>