runtime: expand HACKING.md

This adds high-level descriptions of the scheduler structures, the
user and system stacks, error handling, and synchronization.

Change-Id: I1eed97c6dd4a6e3d351279e967b11c6e64898356
Reviewed-on: https://go-review.googlesource.com/34290Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/HACKING.md

-This is a very incomplete and probably out-of-date guide to
-programming in the Go runtime and how it differs from writing normal
-Go.
+This is a living document and at times it will be out of date. It is
+intended to articulate how programming in the Go runtime differs from
+writing normal Go. It focuses on pervasive concepts rather than
+details of particular interfaces.
+
+Scheduler structures
+====================
+
+The scheduler manages three types of resources that pervade the
+runtime: Gs, Ms, and Ps. It's important to understand these even if
+you're not working on the scheduler.
+
+Gs, Ms, Ps
+----------
+
+A "G" is simply a goroutine. It's represented by type `g`. When a
+goroutine exits, its `g` object is returned to a pool of free `g`s and
+can later be reused for some other goroutine.
+
+An "M" is an OS thread that can be executing user Go code, runtime
+code, a system call, or be idle. It's represented by type `m`. There
+can be any number of Ms at a time since any number of threads may be
+blocked in system calls.
+
+Finally, a "P" represents the resources required to execute user Go
+code, such as scheduler and memory allocator state. It's represented
+by type `p`. There are exactly `GOMAXPROCS` Ps. A P can be thought of
+like a CPU in the OS scheduler and the contents of the `p` type like
+per-CPU state. This is a good place to put state that needs to be
+sharded for efficiency, but doesn't need to be per-thread or
+per-goroutine.
+
+The scheduler's job is to match up a G (the code to execute), an M
+(where to execute it), and a P (the rights and resources to execute
+it). When an M stops executing user Go code, for example by entering a
+system call, it returns its P to the idle P pool. In order to resume
+executing user Go code, for example on return from a system call, it
+must acquire a P from the idle pool.
+
+All `g`, `m`, and `p` objects are heap allocated, but are never freed,
+so their memory remains type stable. As a result, the runtime can
+avoid write barriers in the depths of the scheduler.
+
+User stacks and system stacks
+-----------------------------
+
+Every non-dead G has a *user stack* associated with it, which is what
+user Go code executes on. User stacks start small (e.g., 2K) and grow
+or shrink dynamically.
+
+Every M has a *system stack* associated with it (also known as the M's
+"g0" stack because it's implemented as a stub G) and, on Unix
+platforms, a *signal stack* (also known as the M's "gsignal" stack).
+System and signal stacks cannot grow, but are large enough to execute
+runtime and cgo code (8K in a pure Go binary; system-allocated in a
+cgo binary).
+
+Runtime code often temporarily switches to the system stack using
+`systemstack`, `mcall`, or `asmcgocall` to perform tasks that must not
+be preempted, that must not grow the user stack, or that switch user
+goroutines. Code running on the system stack is implicitly
+non-preemptible and the garbage collector does not scan system stacks.
+While running on the system stack, the current user stack is not used
+for execution.
+
+`getg()` and `getg().m.curg`
+----------------------------
+
+To get the current user `g`, use `getg().m.curg`.
+
+`getg()` alone returns the current `g`, but when executing on the
+system or signal stacks, this will return the current M's "g0" or
+"gsignal", respectively. This is usually not what you want.
+
+To determine if you're running on the user stack or the system stack,
+use `getg() == getg().m.curg`.
+
+Error handling and reporting
+============================
+
+Errors that can reasonably be recovered from in user code should use
+`panic` like usual. However, there are some situations where `panic`
+will cause an immediate fatal error, such as when called on the system
+stack or when called during `mallocgc`.
+
+Most errors in the runtime are not recoverable. For these, use
+`throw`, which dumps the traceback and immediately terminates the
+process. In general, `throw` should be passed a string constant to
+avoid allocating in perilous situations. By convention, additional
+details are printed before `throw` using `print` or `println` and the
+messages are prefixed with "runtime:".
+
+For runtime error debugging, it's useful to run with
+`GOTRACEBACK=system` or `GOTRACEBACK=crash`.
+
+Synchronization
+===============
+
+The runtime has multiple synchronization mechanisms. They differ in
+semantics and, in particular, in whether they interact with the
+goroutine scheduler or the OS scheduler.
+
+The simplest is `mutex`, which is manipulated using `lock` and
+`unlock`. This should be used to protect shared structures for short
+periods. Blocking on a `mutex` directly blocks the M, without
+interacting with the Go scheduler. This means it is safe to use from
+the lowest levels of the runtime, but also prevents any associated G
+and P from being rescheduled.
+
+For one-shot notifications, use `note`, which provides `notesleep` and
+`notewakeup`. Unlike traditional UNIX `sleep`/`wakeup`, `note`s are
+race-free, so `notesleep` returns immediately if the `notewakeup` has
+already happened. A `note` can be reset after use with `noteclear`,
+which must not race with a sleep or wakeup. Like `mutex`, blocking on
+a `note` blocks the M. However, there are different ways to sleep on a
+`note`:`notesleep` also prevents rescheduling of any associated G and
+P, while `notetsleepg` acts like a blocking system call that allows
+the P to be reused to run another G. This is still less efficient than
+blocking the G directly since it consumes an M.
+
+To interact directly with the goroutine scheduler, use `gopark` and
+`goready`. `gopark` parks the current goroutine—putting it in the
+"waiting" state and removing it from the scheduler's run queue—and
+schedules another goroutine on the current M/P. `goready` puts a
+parked goroutine back in the "runnable" state and adds it to the run
+queue.
+
+In summary,
+
+<table>
+<tr><th></th><th colspan="3">Blocks</th></tr>
+<tr><th>Interface</th><th>G</th><th>M</th><th>P</th></tr>
+<tr><td>mutex</td><td>Y</td><td>Y</td><td>Y</td></tr>
+<tr><td>note</td><td>Y</td><td>Y</td><td>Y/N</td></tr>
+<tr><td>park</td><td>Y</td><td>N</td><td>N</td></tr>
+</table>
 
 Unmanaged memory
 ================