Re: [PATCH v1 15/15] perf jevents: Compress the pmu_events_table

[Namhyung Kim]

Hi Ian,

On Thu, Jul 14, 2022 at 11:37 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> The pmu_events array requires 15 pointers per entry which in position
> independent code need relocating. Change the array to be an array of
> offsets within a big C string. Only the offset of the first variable is
> required, subsequent variables are stored in order after the \0
> terminator (requiring a byte per variable rather than 4 bytes per
> offset).
>
> The file size savings are:
> no jevents - the same 19,579,104bytes
> x86 jevents - ~12.5% file size saving 22,088,944bytes vs 25,235,048bytes
> all jevents - ~15.8% file size saving 22,821,896bytes vs 27,106,648bytes
> default build options plus NO_LIBBFD=1.
>
> For example, the x86 build savings come from .rela.dyn and
> .data.rel.ro becoming smaller by 1,911,072bytes and 2,022,656bytes
> respectively. .rodata increases by 789,600bytes, giving an overall
> 3,146,104bytes saving.

Great!  I also noticed my build has fairly large reloc sections.

>
> To make metric strings more shareable, the topic is changed from say
> 'skx metrics' to just 'metrics'.
>
> To try to help with the memory layout the pmu_events are ordered as used
> by perf qsort comparator functions.
>
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
>  tools/perf/pmu-events/jevents.py | 236 +++++++++++++++++++++++++------
>  1 file changed, 193 insertions(+), 43 deletions(-)
>
> diff --git a/tools/perf/pmu-events/jevents.py b/tools/perf/pmu-events/jevents.py
> index d7e03864ca6a..b5bc25790695 100755
> --- a/tools/perf/pmu-events/jevents.py
> +++ b/tools/perf/pmu-events/jevents.py
> @@ -6,8 +6,8 @@ import csv
>  import json
>  import os
>  import sys
> -from typing import Callable
> -from typing import Sequence
> +from typing import (Callable, Dict, Sequence, Set)
> +import collections
>
>  # Global command line arguments.
>  _args = None
> @@ -21,6 +21,19 @@ _arch_std_events = {}
>  _close_table = False
>  # Events to write out when the table is closed
>  _pending_events = []
> +# Global BigCString shared by all structures.
> +_bcs = None
> +# Order specific JsonEvent attributes will be visited.
> +_json_event_attributes = [
> +    # cmp_sevent related attributes.
> +    'name', 'pmu', 'topic', 'desc', 'metric_name', 'metric_group',
> +    # Seems useful, put it early.
> +    'event',
> +    # Short things in alphabetical order.
> +    'aggr_mode', 'compat', 'deprecated', 'perpkg', 'unit',
> +    # Longer things (the last won't be iterated over during decompress).
> +    'metric_constraint', 'metric_expr', 'long_desc'
> +]
>
>
>  def removesuffix(s: str, suffix: str) -> str:
> @@ -40,6 +53,100 @@ def file_name_to_table_name(parents: Sequence[str], dirname: str) -> str:
>    tblname += '_' + dirname
>    return tblname.replace('-', '_')
>
> +def c_len(s: str) -> int:
> +  """Return the length of s a C string
> +
> +  This doesn't handle all escape characters properly. It first assumes
> +  all \ are for escaping, it then adjusts as it will have over counted
> +  \\. The code uses \000 rather than \0 as a terminator as an adjacent
> +  number would be folded into a string of \0 (ie. "\0" + "5" doesn't
> +  equal a terminator followed by the number 5 but the escape of
> +  \05). The code adjusts for \000 but not properly for all octal, hex
> +  or unicode values.
> +  """
> +  try:
> +    utf = s.encode(encoding='utf-8',errors='strict')
> +  except:
> +    print(f'broken string {s}')
> +    raise
> +  return len(utf) - utf.count(b'\\') + utf.count(b'\\\\') - (utf.count(b'\\000') * 2)

Sorry, I don't understand why it needs the last utf.count * 2.
Could you elaborate more?


> +
> +class BigCString:
> +  """A class to hold many strings concatenated together.
> +
> +  Generating a large number of stand-alone C strings creates a large
> +  number of relocations in position independent code. The BigCString
> +  is a helper for this case. It builds a single string which within it
> +  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. 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]
> +  offsets: Dict[str, int]
> +
> +  def __init__(self):
> +    self.strings = set()
> +
> +  def add(self, s: str) -> None:
> +    """Called to add to the big string."""
> +    self.strings.add(s)
> +
> +  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])

I think some blank lines would increase readability a bit.

IIUC these strings are already concatenated with \\000
by build_c_string(), right?


> +    # 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.
> +    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

That means the best pos is the last match?  I guess python
string comparison can deal with strings with NUL bytes in it.

Also I'm not sure how much it actually hits?

In my understanding, each string contains a name, description
and many other fields.  Does it have some duplication?

Maybe I'm missing but not sure it's worth the complexity.

Thanks,
Namhyung


> +      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
> +    # in Python and so appending to one causes memory issues, while
> +    # lists are mutable.
> +    big_string_offset = 0
> +    self.big_string = []
> +    self.offsets = {}
> +    # Emit all strings that aren't folded in a sorted manner.
> +    for s in sorted(self.strings):
> +      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()
>
>  class JsonEvent:
>    """Representation of an event loaded from a json file dictionary."""
> @@ -203,26 +310,18 @@ class JsonEvent:
>          s += f'\t{attr} = {value},\n'
>      return s + '}'
>
> +  def build_c_string(self):
> +    s = ''
> +    for attr in _json_event_attributes:
> +      x = getattr(self, attr)
> +      s += f'{x}\\000' if x else '\\000'
> +    return s
> +
>    def to_c_string(self) -> str:
>      """Representation of the event as a C struct initializer."""
>
> -    def attr_string(attr: str, value: str) -> str:
> -      return f'\t.{attr} = \"{value}\",\n'
> -
> -    def str_if_present(self, attr: str) -> str:
> -      if not getattr(self, attr):
> -        return ''
> -      return attr_string(attr, getattr(self, attr))
> -
> -    s = '{\n'
> -    for attr in [
> -        'aggr_mode', 'compat', 'deprecated', 'desc', 'event', 'long_desc',
> -        'metric_constraint', 'metric_expr', 'metric_group', 'metric_name',
> -        'name', 'perpkg', 'pmu', 'topic', 'unit'
> -    ]:
> -      s += str_if_present(self, attr)
> -    s += '},\n'
> -    return s
> +    s = self.build_c_string()
> +    return f'{{ { _bcs.offsets[s] } }}, /* {s} */\n'
>
>
>  def read_json_events(path: str, topic: str) -> Sequence[JsonEvent]:
> @@ -237,7 +336,6 @@ def read_json_events(path: str, topic: str) -> Sequence[JsonEvent]:
>      event.topic = topic
>    return result
>
> -
>  def preprocess_arch_std_files(archpath: str) -> None:
>    """Read in all architecture standard events."""
>    global _arch_std_events
> @@ -253,7 +351,7 @@ def print_events_table_prefix(tblname: str) -> None:
>    global _close_table
>    if _close_table:
>      raise IOError('Printing table prefix but last table has no suffix')
> -  _args.output_file.write(f'static const struct pmu_event {tblname}[] = {{\n')
> +  _args.output_file.write(f'static const struct compact_pmu_event {tblname}[] = {{\n')
>    _close_table = True
>
>
> @@ -286,23 +384,37 @@ def print_events_table_suffix() -> None:
>      _args.output_file.write(event.to_c_string())
>      _pending_events = []
>
> -  _args.output_file.write("""{
> -\t.name = 0,
> -\t.event = 0,
> -\t.desc = 0,
> -},
> -};
> -""")
> +  _args.output_file.write('};\n\n')
>    _close_table = False
>
> +def get_topic(topic: str) -> str:
> +  if topic.endswith('metrics.json'):
> +    return 'metrics'
> +  return removesuffix(topic, '.json').replace('-', ' ')
> +
> +def preprocess_one_file(parents: Sequence[str], item: os.DirEntry) -> None:
> +
> +  if item.is_dir():
> +    return
> +
> +  # base dir or too deep
> +  level = len(parents)
> +  if level == 0 or level > 4:
> +    return
> +
> +  # Ignore other directories. If the file name does not have a .json
> +  # extension, ignore it. It could be a readme.txt for instance.
> +  if not item.is_file() or not item.name.endswith('.json'):
> +    return
> +
> +  topic = get_topic(item.name)
> +  for event in read_json_events(item.path, topic):
> +    _bcs.add(event.build_c_string())
>
>  def process_one_file(parents: Sequence[str], item: os.DirEntry) -> None:
>    """Process a JSON file during the main walk."""
>    global _sys_event_tables
>
> -  def get_topic(topic: str) -> str:
> -    return removesuffix(topic, '.json').replace('-', ' ')
> -
>    def is_leaf_dir(path: str) -> bool:
>      for item in os.scandir(path):
>        if item.is_dir():
> @@ -337,7 +449,8 @@ def print_mapping_table(archs: Sequence[str]) -> None:
>    _args.output_file.write("""
>  /* Struct used to make the PMU event table implementation opaque to callers. */
>  struct pmu_events_table {
> -        const struct pmu_event *entries;
> +        const struct compact_pmu_event *entries;
> +        size_t length;
>  };
>
>  /*
> @@ -365,7 +478,10 @@ const struct pmu_events_map pmu_events_map[] = {
>        _args.output_file.write("""{
>  \t.arch = "testarch",
>  \t.cpuid = "testcpu",
> -\t.table = { pme_test_soc_cpu },
> +\t.table = {
> +\t.entries = pme_test_soc_cpu,
> +\t.length = ARRAY_SIZE(pme_test_soc_cpu),
> +\t}
>  },
>  """)
>      else:
> @@ -380,7 +496,10 @@ const struct pmu_events_map pmu_events_map[] = {
>              _args.output_file.write(f"""{{
>  \t.arch = "{arch}",
>  \t.cpuid = "{cpuid}",
> -\t.table = {{ {tblname} }}
> +\t.table = {{
> +\t\t.entries = {tblname},
> +\t\t.length = ARRAY_SIZE({tblname})
> +\t}}
>  }},
>  """)
>            first = False
> @@ -388,7 +507,7 @@ const struct pmu_events_map pmu_events_map[] = {
>    _args.output_file.write("""{
>  \t.arch = 0,
>  \t.cpuid = 0,
> -\t.table = { 0 },
> +\t.table = { 0, 0 },
>  }
>  };
>  """)
> @@ -406,23 +525,41 @@ static const struct pmu_sys_events pmu_sys_event_tables[] = {
>  """)
>    for tblname in _sys_event_tables:
>      _args.output_file.write(f"""\t{{
> -\t\t.table = {{ {tblname} }},
> +\t\t.table = {{
> +\t\t\t.entries = {tblname},
> +\t\t\t.length = ARRAY_SIZE({tblname})
> +\t\t}},
>  \t\t.name = \"{tblname}\",
>  \t}},
>  """)
>    _args.output_file.write("""\t{
> -\t\t.table = { 0 }
> +\t\t.table = { 0, 0 }
>  \t},
>  };
>
> -int pmu_events_table_for_each_event(const struct pmu_events_table *table, pmu_event_iter_fn fn,
> +static void decompress(int offset, struct pmu_event *pe)
> +{
> +\tconst char *p = &big_c_string[offset];
> +""")
> +  for attr in _json_event_attributes:
> +    _args.output_file.write(f"""
> +\tpe->{attr} = (*p == '\\0' ? NULL : p);
> +""")
> +    if attr == _json_event_attributes[-1]:
> +      continue
> +    _args.output_file.write('\twhile (*p++);')
> +  _args.output_file.write("""}
> +
> +int pmu_events_table_for_each_event(const struct pmu_events_table *table,
> +                                    pmu_event_iter_fn fn,
>                                      void *data)
>  {
> -        for (const struct pmu_event *pe = &table->entries[0];
> -             pe->name || pe->metric_group || pe->metric_name;
> -             pe++) {
> -                int ret = fn(pe, table, data);
> +        for (size_t i = 0; i < table->length; i++) {
> +                struct pmu_event pe;
> +                int ret;
>
> +                decompress(table->entries[i].offset, &pe);
> +                ret = fn(&pe, table, data);
>                  if (ret)
>                          return ret;
>          }
> @@ -531,7 +668,7 @@ def main() -> None:
>        help='Root of tree containing architecture directories containing json files'
>    )
>    ap.add_argument(
> -      'output_file', type=argparse.FileType('w'), nargs='?', default=sys.stdout)
> +      'output_file', type=argparse.FileType('w', encoding='utf-8'), nargs='?', default=sys.stdout)
>    _args = ap.parse_args()
>
>    _args.output_file.write("""
> @@ -541,6 +678,10 @@ def main() -> None:
>  #include <string.h>
>  #include <stddef.h>
>
> +struct compact_pmu_event {
> +  int offset;
> +};
> +
>  """)
>    archs = []
>    for item in os.scandir(_args.starting_dir):
> @@ -556,6 +697,15 @@ def main() -> None:
>    for arch in archs:
>      arch_path = f'{_args.starting_dir}/{arch}'
>      preprocess_arch_std_files(arch_path)
> +    ftw(arch_path, [], preprocess_one_file)
> +
> +  _bcs.compute()
> +  _args.output_file.write('static const char *const big_c_string =\n')
> +  for s in _bcs.big_string:
> +    _args.output_file.write(s)
> +  _args.output_file.write(';\n\n')
> +  for arch in archs:
> +    arch_path = f'{_args.starting_dir}/{arch}'
>      ftw(arch_path, [], process_one_file)
>      print_events_table_suffix()
>
> --
> 2.37.0.170.g444d1eabd0-goog
>