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Subject[PATCH v4 00/34] Printbufs - new data structure for building strings
Date
Previous discussions:
https://lore.kernel.org/all/20220419203202.2670193-1-kent.overstreet@gmail.com/
https://lore.kernel.org/all/20220519172421.162394-1-kent.overstreet@gmail.com/
https://lore.kernel.org/all/20220604193042.1674951-1-kent.overstreet@gmail.com/

Git repo:
https://evilpiepirate.org/git/bcachefs.git/log/?h=printbuf_v4

Changes since v3:
Bugfixes and performance improvements, the latest iteration of this patch
series has been baking in the bcachefs tree and that shook out some bugs.

Rasmus pointed out that -fno-strict-aliasing is going to cause gcc to generate
nasty code, and indeed it unfortunately does but according to worst case
scenario microbenchmarks it's not a problem for actual performance. Using
memcpy() and memset() in the printbuf helpers _was_ a problem for performance,
so that's been fixed.

-----------

Core idea: Wouldn't it be nice if we had a common data structure and calling
convention for outputting strings?

The core concept this patch series is aimed at cleaning up and standardizing is
that of a "pretty-printer", which is now a function like prt_foo() or
foo_to_text():

void foo_to_text(struct printbuf *out, struct foo)

What this patch series does or enables:

- It becomes quite a bit easier to write composable pretty printers! This is
huge.

- A ton of code that works in terms of raw char * pointers and lengths
(snprintf style, and many weird variations) gets cleaned up, with error prone
raw pointers arithmetic replaced by proper helpers

- A ton of code that emits either directly via printk() or to other places
(sysfs, debugfs) can now output to printbufs, and becomes more reusable and
composable

- Countesy of Matthew Wilcox, the new and very cool %pf() format string, which
allows passing a pretty printer function and its arguments to sprintf() and
family. This means we can now call type specific pretty-printers without
adding them to lib/vsprintf.c and writing a bunch of crazy
parsing-and-dispatch code. For example,

printk("%pd", dentry);

becomes

printk("%pf(%p)", prt_dentry, dentry);

My OOM debugging & reporting patch series that builds off of this uses this
to solve a very real problem that Michal Hocko brought up at LSF - with this
we write shrinkers_to_text(), slab_to_text() which can _also_ now be used for
reporting in debugfs (which Roman has been working on), as well as in the
show_mem() report - the "%pf()" syntax lets us print the output of those
functions without allocating (and having to preallocate) a separate buffer.

- Some new formatting helpers:

Nicely aligned text is much easier to read, and something that we want a
_lot, but outputting nicely aligned text with printf() is a pain in the ass.
Printbufs add tabstops, which can be used for right or left justification -
simple, easy. prt_tab() emits spaces up to the next tabstop, prt_tab_rjust()
advances to the next tabstop right justifying text since the previous
tabstop.

Printbufs also add an indent level, obeyed by prt_newline() which can be very
useful for multi line output.

In the future, \n and \t in format strings may learn to obey these as well.

- Optional heap allocation - no need to statically allocate buffers on the
stack and guess at the output size.

- Lots of consolidating and refactoring

This series replaces seq_buf, which does basically what an earlier version of
printbufs did.

A good chunk of lib/string_helpers.c, as well as lib/hexdump.c are converted
(and simplified!).

Pretty printers in lib/vsprintf.c previously outputted to buffers on the
stack and then copied _that_ to the actual output buffer, that's all gone
(replaced by proper helpers for outputting chars and strings), and they also
used printf_spec for argument passing in ad-hoc ways. This patch series does
a lot towards converting them to more standard pretty printers that can be
called via %pf() instead of having to live in lib/vsprintf.c. Still to do:
format string decoding for argument passing is a mess that's scattered all
over the place.

In the course of working on this patch series, I've spotted a _lot_ more
consolidation and refactoring that needs to be done - we've got a ton of API
fragmentation leading to lots of code duplication.

But I'm already really excited about what this patch series enables.

Cheers!

Kent Overstreet (34):
lib/printbuf: New data structure for printing strings
lib/string_helpers: Convert string_escape_mem() to printbuf
vsprintf: Convert to printbuf
lib/hexdump: Convert to printbuf
vsprintf: %pf(%p)
lib/string_helpers: string_get_size() now returns characters wrote
lib/printbuf: Heap allocation
lib/printbuf: Tabstops, indenting
lib/printbuf: Unit specifiers
lib/pretty-printers: prt_string_option(), prt_bitflags()
vsprintf: Improve number()
vsprintf: prt_u64_minwidth(), prt_u64()
test_printf: Drop requirement that sprintf not write past nul
vsprintf: Start consolidating printf_spec handling
vsprintf: Refactor resource_string()
vsprintf: Refactor fourcc_string()
vsprintf: Refactor ip_addr_string()
vsprintf: Refactor mac_address_string()
vsprintf: time_and_date() no longer takes printf_spec
vsprintf: flags_string() no longer takes printf_spec
vsprintf: Refactor device_node_string, fwnode_string
vsprintf: Refactor hex_string, bitmap_string_list, bitmap_string
Input/joystick/analog: Convert from seq_buf -> printbuf
mm/memcontrol.c: Convert to printbuf
clk: tegra: bpmp: Convert to printbuf
tools/testing/nvdimm: Convert to printbuf
powerpc: Convert to printbuf
x86/resctrl: Convert to printbuf
PCI/P2PDMA: Convert to printbuf
tracing: trace_events_synth: Convert to printbuf
d_path: prt_path()
ACPI/APEI: Add missing include
tracing: Convert to printbuf
Delete seq_buf

Documentation/core-api/printk-formats.rst | 22 +
arch/powerpc/kernel/process.c | 16 +-
arch/powerpc/kernel/security.c | 75 +-
arch/powerpc/platforms/pseries/papr_scm.c | 34 +-
arch/x86/kernel/cpu/resctrl/rdtgroup.c | 16 +-
drivers/acpi/apei/erst-dbg.c | 1 +
drivers/clk/tegra/clk-bpmp.c | 21 +-
drivers/input/joystick/analog.c | 23 +-
drivers/pci/p2pdma.c | 21 +-
fs/d_path.c | 35 +
include/linux/dcache.h | 1 +
include/linux/kernel.h | 12 +
include/linux/pretty-printers.h | 10 +
include/linux/printbuf.h | 253 +++
include/linux/seq_buf.h | 162 --
include/linux/string.h | 5 +
include/linux/string_helpers.h | 8 +-
include/linux/trace_events.h | 2 +-
include/linux/trace_seq.h | 17 +-
kernel/trace/trace.c | 45 +-
kernel/trace/trace_dynevent.c | 34 +-
kernel/trace/trace_events_filter.c | 2 +-
kernel/trace/trace_events_synth.c | 32 +-
kernel/trace/trace_functions_graph.c | 6 +-
kernel/trace/trace_kprobe.c | 2 +-
kernel/trace/trace_seq.c | 111 +-
lib/Makefile | 4 +-
lib/hexdump.c | 246 +--
lib/pretty-printers.c | 60 +
lib/printbuf.c | 253 +++
lib/seq_buf.c | 397 -----
lib/string_helpers.c | 224 +--
lib/test_hexdump.c | 30 +-
lib/test_printf.c | 33 +-
lib/vsprintf.c | 1723 ++++++++++-----------
mm/memcontrol.c | 68 +-
tools/testing/nvdimm/test/ndtest.c | 22 +-
37 files changed, 2050 insertions(+), 1976 deletions(-)
create mode 100644 include/linux/pretty-printers.h
create mode 100644 include/linux/printbuf.h
delete mode 100644 include/linux/seq_buf.h
create mode 100644 lib/pretty-printers.c
create mode 100644 lib/printbuf.c
delete mode 100644 lib/seq_buf.c

--
2.36.1

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