perf jevents: Fold strings optimization

If a shorter string ends a longer string then the shorter string may
reuse the longer string at an offset. For example, on x86 the event
arith.cycles_div_busy and cycles_div_busy can be folded, even though
they have difference names the strings are identical after 6
characters. cycles_div_busy can reuse the arith.cycles_div_busy string
at an offset of 6.

In pmu-events.c this looks like the following where the 'also:' lists
folded strings:

/* offset=177541 */ "arith.cycles_div_busy\000\000pipeline\000Cycles the divider is busy\000\000\000event=0x14,period=2000000,umask=0x1\000\000\000\000\000\000\000\000\000" /* also: cycles_div_busy\000\000pipeline\000Cycles the divider is busy\000\000\000event=0x14,period=2000000,umask=0x1\000\000\000\000\000\000\000\000\000 */

As jevents.py combines multiple strings for an event into a larger
string, the amount of folding is minimal as all parts of the event must
align. Other organizations can benefit more from folding, but lose space
by say recording more offsets.

Signed-off-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: John Garry <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ravi Bangoria <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Xing Zhengjun <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: captain5050

tools/perf/pmu-events/jevents.py

@@ -80,7 +80,9 @@ class BigCString:
   are all the other C strings (to avoid memory issues the string
   itself is held as a list of strings). The offsets within the big
   string are recorded and when stored to disk these don't need
-  relocation.
+  relocation. To reduce the size of the string further, identical
+  strings are merged. If a longer string ends-with the same value as a
+  shorter string, these entries are also merged.
   """
   strings: Set[str]
   big_string: Sequence[str]
@@ -96,6 +98,33 @@ def add(self, s: str) -> None:
   def compute(self) -> None:
     """Called once all strings are added to compute the string and offsets."""
 
+    folded_strings = {}
+    # Determine if two strings can be folded, ie. let 1 string use the
+    # end of another. First reverse all strings and sort them.
+    sorted_reversed_strings = sorted([x[::-1] for x in self.strings])
+
+    # Strings 'xyz' and 'yz' will now be [ 'zy', 'zyx' ]. Scan forward
+    # for each string to see if there is a better candidate to fold it
+    # into, in the example rather than using 'yz' we can use'xyz' at
+    # an offset of 1. We record which string can be folded into which
+    # in folded_strings, we don't need to record the offset as it is
+    # trivially computed from the string lengths.
+    for pos,s in enumerate(sorted_reversed_strings):
+      best_pos = pos
+      for check_pos in range(pos + 1, len(sorted_reversed_strings)):
+        if sorted_reversed_strings[check_pos].startswith(s):
+          best_pos = check_pos
+        else:
+          break
+      if pos != best_pos:
+        folded_strings[s[::-1]] = sorted_reversed_strings[best_pos][::-1]
+
+    # Compute reverse mappings for debugging.
+    fold_into_strings = collections.defaultdict(set)
+    for key, val in folded_strings.items():
+      if key != val:
+        fold_into_strings[val].add(key)
+
     # big_string_offset is the current location within the C string
     # being appended to - comments, etc. don't count. big_string is
     # the string contents represented as a list. Strings are immutable
@@ -104,13 +133,25 @@ def compute(self) -> None:
     big_string_offset = 0
     self.big_string = []
     self.offsets = {}
-    # Emit all strings in a sorted manner.
+
+    # Emit all strings that aren't folded in a sorted manner.
     for s in sorted(self.strings):
-      self.offsets[s] = big_string_offset
-      self.big_string.append(f'/* offset={big_string_offset} */ "')
-      self.big_string.append(s)
-      self.big_string.append('"\n')
-      big_string_offset += c_len(s)
+      if s not in folded_strings:
+        self.offsets[s] = big_string_offset
+        self.big_string.append(f'/* offset={big_string_offset} */ "')
+        self.big_string.append(s)
+        self.big_string.append('"')
+        if s in fold_into_strings:
+          self.big_string.append(' /* also: ' + ', '.join(fold_into_strings[s]) + ' */')
+        self.big_string.append('\n')
+        big_string_offset += c_len(s)
+        continue
+
+    # Compute the offsets of the folded strings.
+    for s in folded_strings.keys():
+      assert s not in self.offsets
+      folded_s = folded_strings[s]
+      self.offsets[s] = self.offsets[folded_s] + c_len(folded_s) - c_len(s)
 
 _bcs = BigCString()