tor-browser

BaseProfilingStack.h (20214B)

---

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef BaseProfilingStack_h
#define BaseProfilingStack_h

#ifndef MOZ_GECKO_PROFILER
#  error Do not #include this header when MOZ_GECKO_PROFILER is not #defined.
#endif

#include "mozilla/Assertions.h"
#include "mozilla/Atomics.h"
#include "mozilla/BaseProfilingCategory.h"

#include <stdint.h>

// This file defines the classes ProfilingStack and ProfilingStackFrame.
// The ProfilingStack manages an array of ProfilingStackFrames.
// It keeps track of the "label stack" and the JS interpreter stack.
// The two stack types are interleaved.
//
// Usage:
//
//  ProfilingStack* profilingStack = ...;
//
//  // For label frames:
//  profilingStack->pushLabelFrame(...);
//  // Execute some code. When finished, pop the frame:
//  profilingStack->pop();
//
//  // For JS stack frames:
//  profilingStack->pushJSFrame(...);
//  // Execute some code. When finished, pop the frame:
//  profilingStack->pop();
//
//
// Concurrency considerations
//
// A thread's profiling stack (and the frames inside it) is only modified by
// that thread. However, the profiling stack can be *read* by a different
// thread, the sampler thread: Whenever the profiler wants to sample a given
// thread A, the following happens:
//  (1) Thread A is suspended.
//  (2) The sampler thread (thread S) reads the ProfilingStack of thread A,
//      including all ProfilingStackFrames that are currently in that stack
//      (profilingStack->frames[0..profilingStack->stackSize()]).
//  (3) Thread A is resumed.
//
// Thread suspension is achieved using platform-specific APIs; refer to each
// platform's Sampler::SuspendAndSampleAndResumeThread implementation in
// platform-*.cpp for details.
//
// When the thread is suspended, the values in profilingStack->stackPointer and
// in the stack frame range
// profilingStack->frames[0..profilingStack->stackPointer] need to be in a
// consistent state, so that thread S does not read partially- constructed stack
// frames. More specifically, we have two requirements:
//  (1) When adding a new frame at the top of the stack, its ProfilingStackFrame
//      data needs to be put in place *before* the stackPointer is incremented,
//      and the compiler + CPU need to know that this order matters.
//  (2) When popping an frame from the stack and then preparing the
//      ProfilingStackFrame data for the next frame that is about to be pushed,
//      the decrement of the stackPointer in pop() needs to happen *before* the
//      ProfilingStackFrame for the new frame is being popuplated, and the
//      compiler + CPU need to know that this order matters.
//
// We can express the relevance of these orderings in multiple ways.
// Option A is to make stackPointer an atomic with SequentiallyConsistent
// memory ordering. This would ensure that no writes in thread A would be
// reordered across any writes to stackPointer, which satisfies requirements
// (1) and (2) at the same time. Option A is the simplest.
// Option B is to use ReleaseAcquire memory ordering both for writes to
// stackPointer *and* for writes to ProfilingStackFrame fields. Release-stores
// ensure that all writes that happened *before this write in program order* are
// not reordered to happen after this write. ReleaseAcquire ordering places no
// requirements on the ordering of writes that happen *after* this write in
// program order.
// Using release-stores for writes to stackPointer expresses requirement (1),
// and using release-stores for writes to the ProfilingStackFrame fields
// expresses requirement (2).
//
// Option B is more complicated than option A, but has much better performance
// on x86/64: In a microbenchmark run on a Macbook Pro from 2017, switching
// from option A to option B reduced the overhead of pushing+popping a
// ProfilingStackFrame by 10 nanoseconds.
// On x86/64, release-stores require no explicit hardware barriers or lock
// instructions.
// On ARM/64, option B may be slower than option A, because the compiler will
// generate hardware barriers for every single release-store instead of just
// for the writes to stackPointer. However, the actual performance impact of
// this has not yet been measured on ARM, so we're currently using option B
// everywhere. This is something that we may want to change in the future once
// we've done measurements.

namespace mozilla {
namespace baseprofiler {

// A call stack can be specified to the JS engine such that all JS entry/exits
// to functions push/pop a stack frame to/from the specified stack.
//
// For more detailed information, see vm/GeckoProfiler.h.
//
class ProfilingStackFrame {
 // A ProfilingStackFrame represents either a label frame or a JS frame.

 // WARNING WARNING WARNING
 //
 // All the fields below are Atomic<...,ReleaseAcquire>. This is needed so
 // that writes to these fields are release-writes, which ensures that
 // earlier writes in this thread don't get reordered after the writes to
 // these fields. In particular, the decrement of the stack pointer in
 // ProfilingStack::pop() is a write that *must* happen before the values in
 // this ProfilingStackFrame are changed. Otherwise, the sampler thread might
 // see an inconsistent state where the stack pointer still points to a
 // ProfilingStackFrame which has already been popped off the stack and whose
 // fields have now been partially repopulated with new values.
 // See the "Concurrency considerations" paragraph at the top of this file
 // for more details.

 // Descriptive label for this stack frame. Must be a static string! Can be
 // an empty string, but not a null pointer.
 Atomic<const char*, ReleaseAcquire> label_;

 // An additional descriptive string of this frame which is combined with
 // |label_| in profiler output. Need not be (and usually isn't) static. Can
 // be null.
 Atomic<const char*, ReleaseAcquire> dynamicString_;

 // Stack pointer for non-JS stack frames, the script pointer otherwise.
 Atomic<void*, ReleaseAcquire> spOrScript;

 // ID of the JS Realm for JS stack frames.
 // Must not be used on non-JS frames; it'll contain either the default 0,
 // or a leftover value from a previous JS stack frame that was using this
 // ProfilingStackFrame object.
 mozilla::Atomic<uint64_t, mozilla::ReleaseAcquire> realmID_;

 // The bytecode offset for JS stack frames.
 // Must not be used on non-JS frames; it'll contain either the default 0,
 // or a leftover value from a previous JS stack frame that was using this
 // ProfilingStackFrame object.
 Atomic<int32_t, ReleaseAcquire> pcOffsetIfJS_;

 // Bits 0...8 hold the Flags. Bits 9...31 hold the category pair.
 Atomic<uint32_t, ReleaseAcquire> flagsAndCategoryPair_;

public:
 ProfilingStackFrame() = default;
 ProfilingStackFrame& operator=(const ProfilingStackFrame& other) {
   label_ = other.label();
   dynamicString_ = other.dynamicString();
   void* spScript = other.spOrScript;
   spOrScript = spScript;
   int32_t offsetIfJS = other.pcOffsetIfJS_;
   pcOffsetIfJS_ = offsetIfJS;
   int64_t realmID = other.realmID_;
   realmID_ = realmID;
   uint32_t flagsAndCategory = other.flagsAndCategoryPair_;
   flagsAndCategoryPair_ = flagsAndCategory;
   return *this;
 }

 // Reserve up to 16 bits for flags, and 16 for category pair.
 enum class Flags : uint32_t {
   // The first three flags describe the kind of the frame and are
   // mutually exclusive. (We still give them individual bits for
   // simplicity.)

   // A regular label frame. These usually come from AutoProfilerLabel.
   IS_LABEL_FRAME = 1 << 0,

   // A special frame indicating the start of a run of JS profiling stack
   // frames. IS_SP_MARKER_FRAME frames are ignored, except for the sp
   // field. These frames are needed to get correct ordering between JS
   // and LABEL frames because JS frames don't carry sp information.
   // SP is short for "stack pointer".
   IS_SP_MARKER_FRAME = 1 << 1,

   // A JS frame.
   IS_JS_FRAME = 1 << 2,

   // An interpreter JS frame that has OSR-ed into baseline. IS_JS_FRAME
   // frames can have this flag set and unset during their lifetime.
   // JS_OSR frames are ignored.
   JS_OSR = 1 << 3,

   // The next three are mutually exclusive.
   // By default, for profiling stack frames that have both a label and a
   // dynamic string, the two strings are combined into one string of the
   // form "<label> <dynamicString>" during JSON serialization. The
   // following flags can be used to change this preset.
   STRING_TEMPLATE_METHOD = 1 << 4,  // "<label>.<dynamicString>"
   STRING_TEMPLATE_GETTER = 1 << 5,  // "get <label>.<dynamicString>"
   STRING_TEMPLATE_SETTER = 1 << 6,  // "set <label>.<dynamicString>"

   // If set, causes this stack frame to be marked as "relevantForJS" in
   // the profile JSON, which will make it show up in the "JS only" call
   // tree view.
   RELEVANT_FOR_JS = 1 << 7,

   // If set, causes the label on this ProfilingStackFrame to be ignored
   // and to be replaced by the subcategory's label.
   LABEL_DETERMINED_BY_CATEGORY_PAIR = 1 << 8,

   // Frame dynamic string does not contain user data.
   NONSENSITIVE = 1 << 9,

   // A JS Baseline Interpreter frame.
   IS_BLINTERP_FRAME = 1 << 10,

   FLAGS_BITCOUNT = 16,
   FLAGS_MASK = (1 << FLAGS_BITCOUNT) - 1
 };

 static_assert(
     uint32_t(ProfilingCategoryPair::LAST) <=
         (UINT32_MAX >> uint32_t(Flags::FLAGS_BITCOUNT)),
     "Too many category pairs to fit into u32 with together with the "
     "reserved bits for the flags");

 bool isLabelFrame() const {
   return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::IS_LABEL_FRAME);
 }

 bool isSpMarkerFrame() const {
   return uint32_t(flagsAndCategoryPair_) &
          uint32_t(Flags::IS_SP_MARKER_FRAME);
 }

 bool isJsFrame() const {
   return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::IS_JS_FRAME);
 }

 bool isOSRFrame() const {
   return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::JS_OSR);
 }

 void setIsOSRFrame(bool isOSR) {
   if (isOSR) {
     flagsAndCategoryPair_ =
         uint32_t(flagsAndCategoryPair_) | uint32_t(Flags::JS_OSR);
   } else {
     flagsAndCategoryPair_ =
         uint32_t(flagsAndCategoryPair_) & ~uint32_t(Flags::JS_OSR);
   }
 }

 const char* label() const {
   uint32_t flagsAndCategoryPair = flagsAndCategoryPair_;
   if (flagsAndCategoryPair &
       uint32_t(Flags::LABEL_DETERMINED_BY_CATEGORY_PAIR)) {
     auto categoryPair = ProfilingCategoryPair(
         flagsAndCategoryPair >> uint32_t(Flags::FLAGS_BITCOUNT));
     return GetProfilingCategoryPairInfo(categoryPair).mLabel;
   }
   return label_;
 }

 const char* dynamicString() const { return dynamicString_; }

 void initLabelFrame(const char* aLabel, const char* aDynamicString, void* sp,
                     ProfilingCategoryPair aCategoryPair, uint32_t aFlags) {
   label_ = aLabel;
   dynamicString_ = aDynamicString;
   spOrScript = sp;
   // pcOffsetIfJS_ is not set and must not be used on label frames.
   flagsAndCategoryPair_ =
       uint32_t(Flags::IS_LABEL_FRAME) |
       (uint32_t(aCategoryPair) << uint32_t(Flags::FLAGS_BITCOUNT)) | aFlags;
   MOZ_ASSERT(isLabelFrame());
 }

 void initSpMarkerFrame(void* sp) {
   label_ = "";
   dynamicString_ = nullptr;
   spOrScript = sp;
   // pcOffsetIfJS_ is not set and must not be used on sp marker frames.
   flagsAndCategoryPair_ = uint32_t(Flags::IS_SP_MARKER_FRAME) |
                           (uint32_t(ProfilingCategoryPair::OTHER)
                            << uint32_t(Flags::FLAGS_BITCOUNT));
   MOZ_ASSERT(isSpMarkerFrame());
 }

 void initJsFrame(const char* aLabel, const char* aDynamicString,
                  void* /* JSScript* */ aScript, int32_t aOffset,
                  uint64_t aRealmID) {
   label_ = aLabel;
   dynamicString_ = aDynamicString;
   spOrScript = aScript;
   pcOffsetIfJS_ = aOffset;
   realmID_ = aRealmID;
   flagsAndCategoryPair_ =
       uint32_t(Flags::IS_JS_FRAME) | (uint32_t(ProfilingCategoryPair::JS)
                                       << uint32_t(Flags::FLAGS_BITCOUNT));
   MOZ_ASSERT(isJsFrame());
 }

 uint32_t flags() const {
   return uint32_t(flagsAndCategoryPair_) & uint32_t(Flags::FLAGS_MASK);
 }

 ProfilingCategoryPair categoryPair() const {
   return ProfilingCategoryPair(flagsAndCategoryPair_ >>
                                uint32_t(Flags::FLAGS_BITCOUNT));
 }

 uint64_t realmID() const { return realmID_; }

 void* stackAddress() const {
   MOZ_ASSERT(!isJsFrame());
   return spOrScript;
 }

 // Note that the pointer returned might be invalid.
 void* rawScript() const {
   MOZ_ASSERT(isJsFrame());
   return spOrScript;
 }
 void setRawScript(void* aScript) {
   MOZ_ASSERT(isJsFrame());
   spOrScript = aScript;
 }

 int32_t pcOffset() const {
   MOZ_ASSERT(isJsFrame());
   return pcOffsetIfJS_;
 }

 void setPCOffset(int32_t aOffset) {
   MOZ_ASSERT(isJsFrame());
   pcOffsetIfJS_ = aOffset;
 }

 // The offset of a pc into a script's code can actually be 0, so to
 // signify a nullptr pc, use a -1 index. This is checked against in
 // pc() and setPC() to set/get the right pc.
 static const int32_t NullPCOffset = -1;
};

// Each thread has its own ProfilingStack. That thread modifies the
// ProfilingStack, pushing and popping elements as necessary.
//
// The ProfilingStack is also read periodically by the profiler's sampler
// thread. This happens only when the thread that owns the ProfilingStack is
// suspended. So there are no genuine parallel accesses.
//
// However, it is possible for pushing/popping to be interrupted by a periodic
// sample. Because of this, we need pushing/popping to be effectively atomic.
//
// - When pushing a new frame, we increment the stack pointer -- making the new
//   frame visible to the sampler thread -- only after the new frame has been
//   fully written. The stack pointer is Atomic<uint32_t,ReleaseAcquire>, so
//   the increment is a release-store, which ensures that this store is not
//   reordered before the writes of the frame.
//
// - When popping an old frame, the only operation is the decrementing of the
//   stack pointer, which is obviously atomic.
//
class ProfilingStack final {
public:
 ProfilingStack() = default;

 MFBT_API ~ProfilingStack();

 void pushLabelFrame(const char* label, const char* dynamicString, void* sp,
                     ProfilingCategoryPair categoryPair, uint32_t flags = 0) {
   // This thread is the only one that ever changes the value of
   // stackPointer.
   // Store the value of the atomic in a non-atomic local variable so that
   // the compiler won't generate two separate loads from the atomic for
   // the size check and the frames[] array indexing operation.
   uint32_t stackPointerVal = stackPointer;

   if (MOZ_UNLIKELY(stackPointerVal >= capacity)) {
     ensureCapacitySlow();
   }
   frames[stackPointerVal].initLabelFrame(label, dynamicString, sp,
                                          categoryPair, flags);

   // This must happen at the end! The compiler will not reorder this
   // update because stackPointer is Atomic<..., ReleaseAcquire>, so any
   // the writes above will not be reordered below the stackPointer store.
   // Do the read and the write as two separate statements, in order to
   // make it clear that we don't need an atomic increment, which would be
   // more expensive on x86 than the separate operations done here.
   // However, don't use stackPointerVal here; instead, allow the compiler
   // to turn this store into a non-atomic increment instruction which
   // takes up less code size.
   stackPointer = stackPointer + 1;
 }

 void pushSpMarkerFrame(void* sp) {
   uint32_t oldStackPointer = stackPointer;

   if (MOZ_UNLIKELY(oldStackPointer >= capacity)) {
     ensureCapacitySlow();
   }
   frames[oldStackPointer].initSpMarkerFrame(sp);

   // This must happen at the end, see the comment in pushLabelFrame.
   stackPointer = oldStackPointer + 1;
 }

 void pushJsOffsetFrame(const char* label, const char* dynamicString,
                        void* script, int32_t offset, uint64_t aRealmID) {
   // This thread is the only one that ever changes the value of
   // stackPointer. Only load the atomic once.
   uint32_t oldStackPointer = stackPointer;

   if (MOZ_UNLIKELY(oldStackPointer >= capacity)) {
     ensureCapacitySlow();
   }
   frames[oldStackPointer].initJsFrame(label, dynamicString, script, offset,
                                       aRealmID);

   // This must happen at the end, see the comment in pushLabelFrame.
   stackPointer = stackPointer + 1;
 }

 void pop() {
   MOZ_ASSERT(stackPointer > 0);
   // Do the read and the write as two separate statements, in order to
   // make it clear that we don't need an atomic decrement, which would be
   // more expensive on x86 than the separate operations done here.
   // This thread is the only one that ever changes the value of
   // stackPointer.
   uint32_t oldStackPointer = stackPointer;
   stackPointer = oldStackPointer - 1;
 }

 uint32_t stackSize() const { return stackPointer; }
 uint32_t stackCapacity() const { return capacity; }

private:
 // Out of line path for expanding the buffer, since otherwise this would get
 // inlined in every DOM WebIDL call.
 MFBT_API MOZ_COLD void ensureCapacitySlow();

 // No copying.
 ProfilingStack(const ProfilingStack&) = delete;
 void operator=(const ProfilingStack&) = delete;

 // No moving either.
 ProfilingStack(ProfilingStack&&) = delete;
 void operator=(ProfilingStack&&) = delete;

 uint32_t capacity = 0;

public:
 // The pointer to the stack frames, this is read from the profiler thread and
 // written from the current thread.
 //
 // This is effectively a unique pointer.
 Atomic<ProfilingStackFrame*, SequentiallyConsistent> frames{nullptr};

 // This may exceed the capacity, so instead use the stackSize() method to
 // determine the number of valid frames in stackFrames. When this is less
 // than stackCapacity(), it refers to the first free stackframe past the top
 // of the in-use stack (i.e. frames[stackPointer - 1] is the top stack
 // frame).
 //
 // WARNING WARNING WARNING
 //
 // This is an atomic variable that uses ReleaseAcquire memory ordering.
 // See the "Concurrency considerations" paragraph at the top of this file
 // for more details.
 Atomic<uint32_t, ReleaseAcquire> stackPointer{0};
};

class AutoGeckoProfilerEntry;
class GeckoProfilerEntryMarker;
class GeckoProfilerBaselineOSRMarker;

class GeckoProfilerThread {
 friend class AutoGeckoProfilerEntry;
 friend class GeckoProfilerEntryMarker;
 friend class GeckoProfilerBaselineOSRMarker;

 ProfilingStack* profilingStack_;

 // Same as profilingStack_ if the profiler is currently active, otherwise
 // null.
 ProfilingStack* profilingStackIfEnabled_;

public:
 MFBT_API GeckoProfilerThread();

 uint32_t stackPointer() {
   MOZ_ASSERT(infraInstalled());
   return profilingStack_->stackPointer;
 }
 ProfilingStackFrame* stack() { return profilingStack_->frames; }
 ProfilingStack* getProfilingStack() { return profilingStack_; }
 ProfilingStack* getProfilingStackIfEnabled() {
   return profilingStackIfEnabled_;
 }

 /*
  * True if the profiler infrastructure is setup.  Should be true in builds
  * that include profiler support except during early startup or late
  * shutdown.  Unrelated to the presence of the Gecko Profiler addon.
  */
 bool infraInstalled() { return profilingStack_ != nullptr; }

 MFBT_API void setProfilingStack(ProfilingStack* profilingStack, bool enabled);
 void enable(bool enable) {
   profilingStackIfEnabled_ = enable ? profilingStack_ : nullptr;
 }
};

}  // namespace baseprofiler
}  // namespace mozilla

#endif /* BaseProfilingStack_h */