On Tue, Mar 26, 2024 at 08:15:56PM +0100, Johannes Berg wrote:

From: Johannes Berg <redacted>

The way __print_symbolic() works is limited and inefficient
in multiple ways:
 - you can only use it with a static list of symbols, but
   e.g. the SKB dropreasons are now a dynamic list

 - it builds the list in memory _three_ times, so it takes
   a lot of memory:
   - The print_fmt contains the list (since it's passed to
     the macro there). This actually contains the names
     _twice_, which is fixed up at runtime.
   - TRACE_DEFINE_ENUM() puts a 24-byte struct trace_eval_map
     for every entry, plus the string pointed to by it, which
     cannot be deduplicated with the strings in the print_fmt
   - The in-kernel symbolic printing creates yet another list
     of struct trace_print_flags for trace_print_symbols_seq()

 - it also requires runtime fixup during init, which is a lot
   of string parsing due to the print_fmt fixup

Introduce __print_sym() to - over time - replace the old one.
We can easily extend this also to __print_flags later, but I
cared only about the SKB dropreasons for now, which has only
__print_symbolic().

This new __print_sym() requires only a single list of items,
created by TRACE_DEFINE_SYM_LIST(), or can even use another
already existing list by using TRACE_DEFINE_SYM_FNS() with
lookup and show methods.

Then, instead of doing an init-time fixup, just do this at the
time when userspace reads the print_fmt. This way, dynamically
updated lists are possible.

For userspace, nothing actually changes, because the print_fmt
is shown exactly the same way the old __print_symbolic() was.

This adds about 4k .text in my test builds, but that'll be
more than paid for by the actual conversions.

Signed-off-by: Johannes Berg <redacted>

Hi Johannes,

I'm seeing some allmodconfig build problems with this applied on top of
net-next.

In file included from ./include/trace/trace_events.h:27,
                 from ./include/trace/define_trace.h:102,
                 from ./include/trace/events/module.h:134,
                 from kernel/module/main.c:64:
./include/trace/stages/init.h:30: warning: "TRACE_DEFINE_SYM_FNS" redefined
   30 | #define TRACE_DEFINE_SYM_FNS(_symbol_id, _lookup, _show)                \
      |
In file included from ./include/linux/trace_events.h:11,
                 from kernel/module/main.c:14:
./include/linux/tracepoint.h:130: note: this is the location of the previous definition
  130 | #define TRACE_DEFINE_SYM_FNS(...)
      |
./include/trace/stages/init.h:54: warning: "TRACE_DEFINE_SYM_LIST" redefined
   54 | #define TRACE_DEFINE_SYM_LIST(_symbol_id, ...)                          \
      |
./include/linux/tracepoint.h:131: note: this is the location of the previous definition
  131 | #define TRACE_DEFINE_SYM_LIST(...)
      |