src/runtime/tracetime.go

// Copyright 2023 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.

// Trace time and clock.

package runtime

import "internal/goarch"

// Timestamps in trace are produced through either nanotime or cputicks
// and divided by traceTimeDiv. nanotime is used everywhere except on
// platforms where osHasLowResClock is true, because the system clock
// isn't granular enough to get useful information out of a trace in
// many cases.
//
// This makes absolute values of timestamp diffs smaller, and so they are
// encoded in fewer bytes.
//
// The target resolution in all cases is 64 nanoseconds.
// This is based on the fact that fundamentally the execution tracer won't emit
// events more frequently than roughly every 200 ns or so, because that's roughly
// how long it takes to call through the scheduler.
// We could be more aggressive and bump this up to 128 ns while still getting
// useful data, but the extra bit doesn't save us that much and the headroom is
// nice to have.
//
// Hitting this target resolution is easy in the nanotime case: just pick a
// division of 64. In the cputicks case it's a bit more complex.
//
// For x86, on a 3 GHz machine, we'd want to divide by 3*64 to hit our target.
// To keep the division operation efficient, we round that up to 4*64, or 256.
// Given what cputicks represents, we use this on all other platforms except
// for PowerPC.
// The suggested increment frequency for PowerPC's time base register is
// 512 MHz according to Power ISA v2.07 section 6.2, so we use 32 on ppc64
// and ppc64le.
const traceTimeDiv = (1-osHasLowResClockInt)*64 + osHasLowResClockInt*(256-224*(goarch.IsPpc64|goarch.IsPpc64le))

// traceTime represents a timestamp for the trace.
type traceTime uint64

// traceClockNow returns a monotonic timestamp. The clock this function gets
// the timestamp from is specific to tracing, and shouldn't be mixed with other
// clock sources.
//
// nosplit because it's called from exitsyscall, which is nosplit.
//
//go:nosplit
func traceClockNow() traceTime {
	if osHasLowResClock {
		return traceTime(cputicks() / traceTimeDiv)
	}
	return traceTime(nanotime() / traceTimeDiv)
}

// traceClockUnitsPerSecond estimates the number of trace clock units per
// second that elapse.
func traceClockUnitsPerSecond() uint64 {
	if osHasLowResClock {
		// We're using cputicks as our clock, so we need a real estimate.
		return uint64(ticksPerSecond() / traceTimeDiv)
	}
	// Our clock is nanotime, so it's just the constant time division.
	// (trace clock units / nanoseconds) * (1e9 nanoseconds / 1 second)
	return uint64(1.0 / float64(traceTimeDiv) * 1e9)
}

// traceFrequency writes a batch with a single EvFrequency event.
//
// freq is the number of trace clock units per second.
func traceFrequency(gen uintptr) {
	w := unsafeTraceWriter(gen, nil)

	// Ensure we have a place to write to.
	w, _ = w.ensure(1 + traceBytesPerNumber /* traceEvFrequency + frequency */)

	// Write out the string.
	w.byte(byte(traceEvFrequency))
	w.varint(traceClockUnitsPerSecond())

	// Immediately flush the buffer.
	systemstack(func() {
		lock(&trace.lock)
		traceBufFlush(w.traceBuf, gen)
		unlock(&trace.lock)
	})
}