Prevent useless cache invalidation

Sometimes we encounter mmap events from perf that span a larger
region than dwfl will parse. In these situations, we would invalidate
the cache and then rebuild it exactly as before - i.e. we would
just waste time for no gain.

This patch checks whether this is happening and prevents the
cache invalidation to speed up the parse process. In one of my
cases this helps significantly:

Before:
 Performance counter stats for './perfparser --input perf.data --output /dev/null':

      19553.281950      task-clock:u (msec)       #    0.998 CPUs utilized
                 0      context-switches:u        #    0.000 K/sec
                 0      cpu-migrations:u          #    0.000 K/sec
            59,997      page-faults:u             #    0.003 M/sec
    58,567,488,383      cycles:u                  #    2.995 GHz
   158,503,674,356      instructions:u            #    2.71  insn per cycle
    38,712,268,219      branches:u                # 1979.835 M/sec
       193,851,125      branch-misses:u           #    0.50% of all branches

      19.593726259 seconds time elapsed

After:
 Performance counter stats for './perfparser --input perf.data --output /dev/null':

      11775.076614      task-clock:u (msec)       #    0.997 CPUs utilized
                 0      context-switches:u        #    0.000 K/sec
                 0      cpu-migrations:u          #    0.000 K/sec
            22,747      page-faults:u             #    0.002 M/sec
    35,463,053,591      cycles:u                  #    3.012 GHz
    94,541,494,541      instructions:u            #    2.67  insn per cycle
    23,038,811,370      branches:u                # 1956.574 M/sec
       120,291,747      branch-misses:u           #    0.52% of all branches

      11.807795955 seconds time elapsed

Change-Id: I3efdf5941c5f66cb2d38fecc8ef824c6aef245da
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/app/perfsymboltable.cpp b/app/perfsymboltable.cpp
index 0c1a7b3..0bd45b7 100644
--- a/app/perfsymboltable.cpp
+++ b/app/perfsymboltable.cpp
@@ -509,6 +509,14 @@ Dwfl_Module *PerfSymbolTable::module(quint64 addr, const PerfElfMap::ElfInfo &el
 
     Dwfl_Module *mod = dwfl_addrmodule(m_dwfl, addr);
 
+    if (!mod) {
+        // check whether we queried for an address outside the elf range parsed
+        // by dwfl. If that is the case, then we would invalidate the cache and
+        // re-report the library again - essentially recreating the current state
+        // for no gain, except wasting time
+        mod = dwfl_addrmodule(m_dwfl, elf.addr);
+    }
+
     if (mod) {
         // If dwfl has a module and it's not the same as what we want, report the module
         // we want. Many modules overlap ld.so, so if we've reported even one sample from