LuaJIT platform profiler documentation

Introduce a new document on LuaJIT platform profiler

* LuaJIT platform profiler is a new feature implemented in
  Tarantool 2.10.0. The document describes the profiler's behavior
  as of this and next Tarantool versions.
* The document is placed in the Tooling chapter.

Closes #2587

Author: ligurio

doc/tooling/index.rst

@@ -11,4 +11,5 @@ to work with Tarantool.
     tcm/index
     interactive_console
     luajit_memprof
+    luajit_sysprof
     luajit_getmetrics

doc/tooling/luajit_sysprof.rst

@@ -0,0 +1,320 @@
+..  _luajit_sysprof:
+
+LuaJIT platform profiler
+========================
+
+The default profiling options for LuaJIT are not fine enough to
+get an understanding of performance. For example, performance only
+able to show host stack, so all the Lua calls are seen as single
+``pcall()``. Oppositely, the ``jit.p`` module provided with LuaJIT
+is not able to give any information about the host stack.
+
+Starting from version :doc:`2.10.0 </release/2.10.0>`, Tarantool
+has a built‑in module called ``misc.syprof`` that implements a
+LuaJIT sampling profiler (which we will just call *the profiler*
+in this section). The profiler is able to capture both guest and
+host stacks simultaneously, along with virtual machine states, so
+it can show the whole picture.
+
+The following profiling modes are available:
+
+*    **Default**: only virtual machine state counters.
+*    **Leaf**: shows the last frame on the stack.
+*    **Callchain**: performs a complete stack dump.
+
+The profiler comes with the default parser, which produces output in
+a `flamegraph.pl`-suitable format.
+
+..  contents::
+    :local:
+    :depth: 2
+
+.. _profiler_usage:
+
+Working with the profiler
+-------------------------
+
+Usage of the profiler involves two steps:
+
+1.  :ref:`Collecting <profiler_usage_get>` a binary profile of
+    stacks, (further, *binary sampling profile* or *binary profile*
+    for short).
+2.  :ref:`Parsing <profiler_usage_parse>` the collected binary
+    profile to get a human-readable profiling report.
+
+.. _profiler_usage_get:
+
+Collecting binary profile
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To collect a binary profile for a particular part of the Lua and C code,
+you need to place this part between two ``misc.sysprof`` functions,
+namely, ``misc.sysprof.start()`` and ``misc.sysprof.stop()``, and
+then execute the code under Tarantool.
+
+Below is a chunk of Lua code named ``test.lua`` to illustrate this.
+
+.. _profiler_usage_example01:
+
+..  code-block:: lua
+    :linenos:
+
+    local function payload()
+      local function fib(n)
+        if n <= 1 then
+          return n
+        end
+        return fib(n - 1) + fib(n - 2)
+      end
+      return fib(32)
+    end
+
+    payload()
+
+    local res, err = misc.sysprof.start({mode = 'C', interval = 1, path = 'sysprof.bin'})
+    assert(res, err)
+
+    payload()
+
+    res, err = misc.sysprof.stop()
+    assert(res, err)
+
+The Lua code for starting the profiler -- as in line 1 in the
+``test.lua`` example above -- is:
+
+..  code-block:: lua
+
+    local str, err = misc.sysprof.start({mode = 'C', interval = 1, path = 'sysprof.bin'})
+
+where ``mode`` is a profiling mode, ``interval`` is a sampling interval,
+and ``sysprof.bin`` is the name of the binary file where
+profiling events are written.
+
+If the operation fails, for example if it is not possible to open
+a file for writing or if the profiler is already running,
+``misc.sysprof.start()`` returns ``nil`` as the first result,
+an error-message string as the second result,
+and a system-dependent error code number as the third result.
+If the operation succeeds, ``misc.sysprof.start()`` returns ``true``.
+
+The Lua code for stopping the profiler -- as in line 15 in the
+``test.lua`` example above -- is:
+
+..  code-block:: lua
+
+    local res, err = misc.sysprof.stop()
+
+If the operation fails, for example if  there is an error when the
+file descriptor is being closed or if there is a failure during
+reporting, ``misc.sysprof.stop()`` returns ``nil`` as the first
+result, an error-message string as the second result,
+and a system-dependent error code number as the third result.
+If the operation succeeds, ``misc.sysprof.stop()`` returns ``true``.
+
+.. _profiler_usage_generate:
+
+To generate the file with memory profile in binary format
+(in the :ref:`test.lua code example above <profiler_usage_example01>`
+the file name is ``sysprof.bin``), execute the code under Tarantool:
+
+..  code-block:: console
+
+    $ tarantool test.lua
+
+Tarantool collects the allocation events in ``sysprof.bin``, puts
+the file in its :ref:`working directory <cfg_basic-work_dir>`,
+and closes the session.
+
+.. _profiler_usage_parse:
+
+Parsing binary profile and generating profiling report
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. _profiler_usage_parse_command:
+
+After getting the platform profile in binary format, the next step is
+to parse it to get a human-readable profiling report. You can do this
+via Tarantool by using the following command
+(mind the hyphen ``-`` before the filename):
+
+..  code-block:: console
+
+    $ tarantool -e 'require("sysprof")(arg)' - sysprof.bin > tmp
+    $ curl -O https://raw.githubusercontent.com/brendangregg/FlameGraph/refs/heads/master/flamegraph.pl
+    $ perl flamegraph.pl tmp > sysprof.svg
+
+where ``sysprof.bin`` is the binary profile
+:ref:`generated earlier <profiler_usage_generate>` by ``tarantool test.lua``.
+(Warning: there is a slight behavior change here, the ``tarantool -e ...``
+command was slightly different in Tarantool versions prior to Tarantool 2.8.1.)
+Resulted SVG image contains a flamegraph with collected stacks and can be opened
+by modern web-browser for analysis.
+
+As for investigating the Lua code with the help of profiling reports,
+it is always code-dependent and there can't be hundred per cent definite
+recommendations in this regard. Nevertheless, you can see some of the things
+in the :ref:`Profiling report analysis example <profiler_analysis>` later.
+
+.. _profiler_api:
+
+The C API
+~~~~~~~~~
+
+The platform profiler provides a low-level C interface:
+
+..  code-block:: console
+
+    int luaM_sysprof_set_writer(sp_writer writer). Sets writer function for sysprof.
+
+    int luaM_sysprof_set_on_stop(sp_on_stop on_stop). Sets on stop callback for sysprof to clear resources.
+
+    int luaM_sysprof_set_backtracer(sp_backtracer backtracer). Sets backtracking function. If backtracer arg is NULL, the default backtracer is set.
+    There is no need to call the configuration functions multiple times, if you are starting and stopping profiler several times in a single program. Also, it is not necessary to configure sysprof for the default mode, however, one MUST configure it for the other modes.
+
+    int luaM_sysprof_start(lua_State *L, const struct luam_Sysprof_Options *opt)
+
+    int luaM_sysprof_stop(lua_State *L)
+
+    int luaM_sysprof_report(struct luam_Sysprof_Counters *counters). Writes profiling counters for each vmstate.
+
+All of the functions return 0 on success and an error code on failure.
+
+The configuration C types are:
+
+..  code-block:: console
+
+    /* Profiler configurations. */
+    /*
+     ** Writer function for profile events. Must be async-safe, see also
+     ** `man 7 signal-safety`.
+     ** Should return amount of written bytes on success or zero in case of error.
+     ** Setting *data to NULL means end of profiling.
+     ** For details see <lj_wbuf.h>.
+     */
+
+    typedef size_t (*sp_writer)(const void **data, size_t len, void *ctx);
+    /*
+    ** Callback on profiler stopping. Required for correctly cleaning
+    ** at VM finalization when profiler is still running.
+    ** Returns zero on success.
+    */
+    typedef int (*sp_on_stop)(void *ctx, uint8_t *buf);
+    /*
+    ** Backtracing function for the host stack. Should call `frame_writer` on
+    ** each frame in the stack in the order from the stack top to the stack
+    ** bottom. The `frame_writer` function is implemented inside the sysprof
+    ** and will be passed to the `backtracer` function. If `frame_writer` returns
+    ** NULL, backtracing should be stopped. If `frame_writer` returns not NULL,
+    ** the backtracing should be continued if there are frames left.
+    */
+    typedef void (*sp_backtracer)(void *(*frame_writer)(int frame_no, void *addr));
+
+Profiler options are the following:
+
+..  code-block:: console
+
+    struct luam_Sysprof_Options {
+      /* Profiling mode. */
+      uint8_t mode;
+      /* Sampling interval in msec. */
+      uint64_t interval;
+      /* Custom buffer to write data. */
+      uint8_t *buf;
+      /* The buffer's size. */
+      size_t len;
+      /* Context for the profile writer and final callback. */
+      void *ctx;
+    };
+
+Profiling modes:
+
+..  code-block:: console
+
+    /*
+    ** DEFAULT mode collects only data for luam_sysprof_counters, which is stored
+    ** in memory and can be collected with luaM_sysprof_report after profiler
+    ** stops.
+    */
+    #define LUAM_SYSPROF_DEFAULT 0
+    /*
+    ** LEAF mode = DEFAULT + streams samples with only top frames of host and
+    ** guests stacks in format described in <lj_sysprof.h>
+    */
+    #define LUAM_SYSPROF_LEAF 1
+    /*
+    ** CALLGRAPH mode = DEFAULT + streams samples with full callchains of host
+    ** and guest stacks in format described in <lj_sysprof.h>
+    */
+    #define LUAM_SYSPROF_CALLGRAPH 2
+
+Counters structure for the luaM_Sysprof_Report:
+
+..  code-block:: console
+
+   struct luam_Sysprof_Counters {
+     uint64_t vmst_interp;
+     uint64_t vmst_lfunc;
+     uint64_t vmst_ffunc;
+     uint64_t vmst_cfunc;
+     uint64_t vmst_gc;
+     uint64_t vmst_exit;
+     uint64_t vmst_record;
+     uint64_t vmst_opt;
+     uint64_t vmst_asm;
+     uint64_t vmst_trace;
+     /*
+     ** XXX: Order of vmst counters is important: it should be the same as the
+     ** order of the vmstates.
+     */
+     uint64_t samples;
+   };
+
+Caveats:
+
+*    Providing writers, backtracers, etc; in the Default mode is pointless, since
+     it just collect counters.
+*    There is NO default configuration for sysprof, so the ``luaM_Sysprof_Configure``
+     must be called before the first run of the sysprof. Mind the async-safety.
+
+The Lua API
+~~~~~~~~~~~
+
+*   ``misc.sysprof.start(opts)``
+*   ``misc.sysprof.stop()``
+*   ``misc.sysprof.report()``
+
+First two functions return boolean ``res`` and ``err``, which is
+``nil`` on success and contains an error message on failure.
+
+``misc.sysprof.report`` returns a Lua table containing the
+following counters:
+
+..  code-block:: console
+
+    {
+      "samples" = int,
+      "INTERP" = int,
+      "LFUNC" = int,
+      "FFUNC" = int,
+      "CFUNC" = int,
+      "GC" = int,
+      "EXIT" = int,
+      "RECORD" = int,
+      "OPT" = int,
+      "ASM" = int,
+      "TRACE" = int
+   }
+
+Parameter opts for the ``misc.sysprof.start`` can contain the
+following parameters:
+
+..  code-block:: console
+
+   {
+     mode = 'D'/'L'/'C', -- 'D' = DEFAULT, 'L' = LEAF, 'C' = CALLGRAPH
+     interval = 10, -- sampling interval in msec.
+     path = '/path/to/file' -- location to store profile data.
+   }
+
+Mode MUST be provided always, interval and path are optional.
+The default interval is 10 msec, default path is ``sysprof.bin``.