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Diffstat (limited to 'libgo/go/runtime/preempt.go')
| -rw-r--r-- | libgo/go/runtime/preempt.go | 370 |
1 files changed, 370 insertions, 0 deletions
diff --git a/libgo/go/runtime/preempt.go b/libgo/go/runtime/preempt.go new file mode 100644 index 00000000000..1a8f9ac4a3d --- /dev/null +++ b/libgo/go/runtime/preempt.go @@ -0,0 +1,370 @@ +// Copyright 2019 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +// Goroutine preemption +// +// A goroutine can be preempted at any safe-point. Currently, there +// are a few categories of safe-points: +// +// 1. A blocked safe-point occurs for the duration that a goroutine is +// descheduled, blocked on synchronization, or in a system call. +// +// 2. Synchronous safe-points occur when a running goroutine checks +// for a preemption request. +// +// 3. Asynchronous safe-points occur at any instruction in user code +// where the goroutine can be safely paused and a conservative +// stack and register scan can find stack roots. The runtime can +// stop a goroutine at an async safe-point using a signal. +// +// At both blocked and synchronous safe-points, a goroutine's CPU +// state is minimal and the garbage collector has complete information +// about its entire stack. This makes it possible to deschedule a +// goroutine with minimal space, and to precisely scan a goroutine's +// stack. +// +// Synchronous safe-points are implemented by overloading the stack +// bound check in function prologues. To preempt a goroutine at the +// next synchronous safe-point, the runtime poisons the goroutine's +// stack bound to a value that will cause the next stack bound check +// to fail and enter the stack growth implementation, which will +// detect that it was actually a preemption and redirect to preemption +// handling. +// +// Preemption at asynchronous safe-points is implemented by suspending +// the thread using an OS mechanism (e.g., signals) and inspecting its +// state to determine if the goroutine was at an asynchronous +// safe-point. Since the thread suspension itself is generally +// asynchronous, it also checks if the running goroutine wants to be +// preempted, since this could have changed. If all conditions are +// satisfied, it adjusts the signal context to make it look like the +// signaled thread just called asyncPreempt and resumes the thread. +// asyncPreempt spills all registers and enters the scheduler. +// +// (An alternative would be to preempt in the signal handler itself. +// This would let the OS save and restore the register state and the +// runtime would only need to know how to extract potentially +// pointer-containing registers from the signal context. However, this +// would consume an M for every preempted G, and the scheduler itself +// is not designed to run from a signal handler, as it tends to +// allocate memory and start threads in the preemption path.) + +package runtime + +import ( + "runtime/internal/atomic" +) + +type suspendGState struct { + g *g + + // dead indicates the goroutine was not suspended because it + // is dead. This goroutine could be reused after the dead + // state was observed, so the caller must not assume that it + // remains dead. + dead bool + + // stopped indicates that this suspendG transitioned the G to + // _Gwaiting via g.preemptStop and thus is responsible for + // readying it when done. + stopped bool +} + +// suspendG suspends goroutine gp at a safe-point and returns the +// state of the suspended goroutine. The caller gets read access to +// the goroutine until it calls resumeG. +// +// It is safe for multiple callers to attempt to suspend the same +// goroutine at the same time. The goroutine may execute between +// subsequent successful suspend operations. The current +// implementation grants exclusive access to the goroutine, and hence +// multiple callers will serialize. However, the intent is to grant +// shared read access, so please don't depend on exclusive access. +// +// This must be called from the system stack and the user goroutine on +// the current M (if any) must be in a preemptible state. This +// prevents deadlocks where two goroutines attempt to suspend each +// other and both are in non-preemptible states. There are other ways +// to resolve this deadlock, but this seems simplest. +// +// TODO(austin): What if we instead required this to be called from a +// user goroutine? Then we could deschedule the goroutine while +// waiting instead of blocking the thread. If two goroutines tried to +// suspend each other, one of them would win and the other wouldn't +// complete the suspend until it was resumed. We would have to be +// careful that they couldn't actually queue up suspend for each other +// and then both be suspended. This would also avoid the need for a +// kernel context switch in the synchronous case because we could just +// directly schedule the waiter. The context switch is unavoidable in +// the signal case. +// +//go:systemstack +func suspendG(gp *g) suspendGState { + if mp := getg().m; mp.curg != nil && readgstatus(mp.curg) == _Grunning { + // Since we're on the system stack of this M, the user + // G is stuck at an unsafe point. If another goroutine + // were to try to preempt m.curg, it could deadlock. + throw("suspendG from non-preemptible goroutine") + } + + // See https://golang.org/cl/21503 for justification of the yield delay. + const yieldDelay = 10 * 1000 + var nextYield int64 + + // Drive the goroutine to a preemption point. + stopped := false + var asyncM *m + var asyncGen uint32 + var nextPreemptM int64 + for i := 0; ; i++ { + switch s := readgstatus(gp); s { + default: + if s&_Gscan != 0 { + // Someone else is suspending it. Wait + // for them to finish. + // + // TODO: It would be nicer if we could + // coalesce suspends. + break + } + + dumpgstatus(gp) + throw("invalid g status") + + case _Gdead: + // Nothing to suspend. + // + // preemptStop may need to be cleared, but + // doing that here could race with goroutine + // reuse. Instead, goexit0 clears it. + return suspendGState{dead: true} + + case _Gcopystack: + // The stack is being copied. We need to wait + // until this is done. + + case _Gexitingsyscall: + // This is a transient state. Try again. + + case _Gpreempted: + // We (or someone else) suspended the G. Claim + // ownership of it by transitioning it to + // _Gwaiting. + if !casGFromPreempted(gp, _Gpreempted, _Gwaiting) { + break + } + + // We stopped the G, so we have to ready it later. + stopped = true + + s = _Gwaiting + fallthrough + + case _Grunnable, _Gsyscall, _Gwaiting: + // Claim goroutine by setting scan bit. + // This may race with execution or readying of gp. + // The scan bit keeps it from transition state. + if !castogscanstatus(gp, s, s|_Gscan) { + break + } + + // Clear the preemption request. It's safe to + // reset the stack guard because we hold the + // _Gscan bit and thus own the stack. + gp.preemptStop = false + gp.preempt = false + + // The goroutine was already at a safe-point + // and we've now locked that in. + // + // TODO: It would be much better if we didn't + // leave it in _Gscan, but instead gently + // prevented its scheduling until resumption. + // Maybe we only use this to bump a suspended + // count and the scheduler skips suspended + // goroutines? That wouldn't be enough for + // {_Gsyscall,_Gwaiting} -> _Grunning. Maybe + // for all those transitions we need to check + // suspended and deschedule? + return suspendGState{g: gp, stopped: stopped} + + case _Grunning: + // Optimization: if there is already a pending preemption request + // (from the previous loop iteration), don't bother with the atomics. + if asyncM != nil && gp.preemptStop && gp.preempt && asyncM == gp.m && atomic.Load(&asyncM.preemptGen) == asyncGen { + break + } + + // Temporarily block state transitions. + if !castogscanstatus(gp, _Grunning, _Gscanrunning) { + break + } + + // Request synchronous preemption. + gp.preemptStop = true + gp.preempt = true + + // Prepare for asynchronous preemption. + asyncM2 := gp.m + asyncGen2 := atomic.Load(&asyncM2.preemptGen) + needAsync := asyncM != asyncM2 || asyncGen != asyncGen2 + asyncM = asyncM2 + asyncGen = asyncGen2 + + casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning) + + // Send asynchronous preemption. We do this + // after CASing the G back to _Grunning + // because preemptM may be synchronous and we + // don't want to catch the G just spinning on + // its status. + if preemptMSupported && debug.asyncpreemptoff == 0 && needAsync { + // Rate limit preemptM calls. This is + // particularly important on Windows + // where preemptM is actually + // synchronous and the spin loop here + // can lead to live-lock. + now := nanotime() + if now >= nextPreemptM { + nextPreemptM = now + yieldDelay/2 + preemptM(asyncM) + } + } + } + + // TODO: Don't busy wait. This loop should really only + // be a simple read/decide/CAS loop that only fails if + // there's an active race. Once the CAS succeeds, we + // should queue up the preemption (which will require + // it to be reliable in the _Grunning case, not + // best-effort) and then sleep until we're notified + // that the goroutine is suspended. + if i == 0 { + nextYield = nanotime() + yieldDelay + } + if nanotime() < nextYield { + procyield(10) + } else { + osyield() + nextYield = nanotime() + yieldDelay/2 + } + } +} + +// resumeG undoes the effects of suspendG, allowing the suspended +// goroutine to continue from its current safe-point. +func resumeG(state suspendGState) { + if state.dead { + // We didn't actually stop anything. + return + } + + gp := state.g + switch s := readgstatus(gp); s { + default: + dumpgstatus(gp) + throw("unexpected g status") + + case _Grunnable | _Gscan, + _Gwaiting | _Gscan, + _Gsyscall | _Gscan: + casfrom_Gscanstatus(gp, s, s&^_Gscan) + } + + if state.stopped { + // We stopped it, so we need to re-schedule it. + ready(gp, 0, true) + } +} + +// canPreemptM reports whether mp is in a state that is safe to preempt. +// +// It is nosplit because it has nosplit callers. +// +//go:nosplit +func canPreemptM(mp *m) bool { + return mp.locks == 0 && mp.mallocing == 0 && mp.preemptoff == "" && mp.p.ptr().status == _Prunning +} + +//go:generate go run mkpreempt.go + +// asyncPreempt saves all user registers and calls asyncPreempt2. +// +// When stack scanning encounters an asyncPreempt frame, it scans that +// frame and its parent frame conservatively. +// +// asyncPreempt is implemented in assembly. +func asyncPreempt() + +//go:nosplit +func asyncPreempt2() { + gp := getg() + gp.asyncSafePoint = true + if gp.preemptStop { + mcall(preemptPark) + } else { + mcall(gopreempt_m) + } + gp.asyncSafePoint = false +} + +// wantAsyncPreempt returns whether an asynchronous preemption is +// queued for gp. +func wantAsyncPreempt(gp *g) bool { + // Check both the G and the P. + return (gp.preempt || gp.m.p != 0 && gp.m.p.ptr().preempt) && readgstatus(gp)&^_Gscan == _Grunning +} + +// isAsyncSafePoint reports whether gp at instruction PC is an +// asynchronous safe point. This indicates that: +// +// 1. It's safe to suspend gp and conservatively scan its stack and +// registers. There are no potentially hidden pointer values and it's +// not in the middle of an atomic sequence like a write barrier. +// +// 2. gp has enough stack space to inject the asyncPreempt call. +// +// 3. It's generally safe to interact with the runtime, even if we're +// in a signal handler stopped here. For example, there are no runtime +// locks held, so acquiring a runtime lock won't self-deadlock. +func isAsyncSafePoint(gp *g, pc uintptr) bool { + mp := gp.m + + // Only user Gs can have safe-points. We check this first + // because it's extremely common that we'll catch mp in the + // scheduler processing this G preemption. + if mp.curg != gp { + return false + } + + // Check M state. + if mp.p == 0 || !canPreemptM(mp) { + return false + } + + // Check if PC is an unsafe-point. + f := FuncForPC(pc) + if f == nil { + // Not Go code. + return false + } + name := f.Name() + if hasPrefix(name, "runtime.") || + hasPrefix(name, "runtime..z2finternal..z2f") || + hasPrefix(name, "reflect.") { + // For now we never async preempt the runtime or + // anything closely tied to the runtime. Known issues + // include: various points in the scheduler ("don't + // preempt between here and here"), much of the defer + // implementation (untyped info on stack), bulk write + // barriers (write barrier check), + // reflect.{makeFuncStub,methodValueCall}. + // + // TODO(austin): We should improve this, or opt things + // in incrementally. + return false + } + + return true +} |