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

[Ian Rogers]

On Tue, Jul 26, 2022 at 9:04 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> 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?

Yep, so
1) start with the length of the string
2) \ will escape the next character, e.g. \n, so instead of counting
that as 2 count it as 1
3) but, \\ will now get counted as 0 and it should be 1
4) but also, a numeric escape like \000 will get counted as 3 rather
than 1 so adjust that.

Obviously this isn't full C string parsing, but it is sufficient for
what comes in the json files.

>
> > +
> > +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.

Ack. Can add in v2.

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

They are, but this code is agnostic to that. The code must give every
string an offset, but it also tries as much as possible to combine
strings. The \\000 defeats that in all but 1 case for x86, but if we
were to have 1 offset per field it does better.

> > +    # 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?

Once for x86 currently. The code runs quickly so I didn't disable it.
In other situations it hits more, as mentioned above.

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

There may be, and having an offset per variable would mean we could
take advantage of that. What I found was that a majority of the
variables are empty/NULL so what we lose by not having sharing is more
than gained by reducing a variable down to 1 byte rather than 4 (in
the order of 100s of KB).

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

Agreed. There's limited utility in finding 1 string inside another for
the current string representation. The code is still necessary to give
every string an offset. The code to find 1 string inside another isn't
that large nor that slow, so I kept it around.

Thanks,
Ian

> 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
> >