tracetime.go

     1  // Copyright 2023 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Trace time and clock.
     6  
     7  package runtime
     8  
     9  import (
    10  	"internal/goarch"
    11  	_ "unsafe"
    12  )
    13  
    14  // Timestamps in trace are produced through either nanotime or cputicks
    15  // and divided by traceTimeDiv. nanotime is used everywhere except on
    16  // platforms where osHasLowResClock is true, because the system clock
    17  // isn't granular enough to get useful information out of a trace in
    18  // many cases.
    19  //
    20  // This makes absolute values of timestamp diffs smaller, and so they are
    21  // encoded in fewer bytes.
    22  //
    23  // The target resolution in all cases is 64 nanoseconds.
    24  // This is based on the fact that fundamentally the execution tracer won't emit
    25  // events more frequently than roughly every 200 ns or so, because that's roughly
    26  // how long it takes to call through the scheduler.
    27  // We could be more aggressive and bump this up to 128 ns while still getting
    28  // useful data, but the extra bit doesn't save us that much and the headroom is
    29  // nice to have.
    30  //
    31  // Hitting this target resolution is easy in the nanotime case: just pick a
    32  // division of 64. In the cputicks case it's a bit more complex.
    33  //
    34  // For x86, on a 3 GHz machine, we'd want to divide by 3*64 to hit our target.
    35  // To keep the division operation efficient, we round that up to 4*64, or 256.
    36  // Given what cputicks represents, we use this on all other platforms except
    37  // for PowerPC.
    38  // The suggested increment frequency for PowerPC's time base register is
    39  // 512 MHz according to Power ISA v2.07 section 6.2, so we use 32 on ppc64
    40  // and ppc64le.
    41  const traceTimeDiv = (1-osHasLowResClockInt)*64 + osHasLowResClockInt*(256-224*(goarch.IsPpc64|goarch.IsPpc64le))
    42  
    43  // traceTime represents a timestamp for the trace.
    44  type traceTime uint64
    45  
    46  // traceClockNow returns a monotonic timestamp. The clock this function gets
    47  // the timestamp from is specific to tracing, and shouldn't be mixed with other
    48  // clock sources.
    49  //
    50  // nosplit because it's called from exitsyscall, which is nosplit.
    51  //
    52  // traceClockNow is called by golang.org/x/exp/trace using linkname.
    53  //
    54  //go:linkname traceClockNow
    55  //go:nosplit
    56  func traceClockNow() traceTime {
    57  	if osHasLowResClock {
    58  		return traceTime(cputicks() / traceTimeDiv)
    59  	}
    60  	return traceTime(nanotime() / traceTimeDiv)
    61  }
    62  
    63  // traceClockUnitsPerSecond estimates the number of trace clock units per
    64  // second that elapse.
    65  func traceClockUnitsPerSecond() uint64 {
    66  	if osHasLowResClock {
    67  		// We're using cputicks as our clock, so we need a real estimate.
    68  		return uint64(ticksPerSecond() / traceTimeDiv)
    69  	}
    70  	// Our clock is nanotime, so it's just the constant time division.
    71  	// (trace clock units / nanoseconds) * (1e9 nanoseconds / 1 second)
    72  	return uint64(1.0 / float64(traceTimeDiv) * 1e9)
    73  }
    74  
    75  // traceFrequency writes a batch with a single EvFrequency event.
    76  //
    77  // freq is the number of trace clock units per second.
    78  func traceFrequency(gen uintptr) {
    79  	w := unsafeTraceWriter(gen, nil)
    80  
    81  	// Ensure we have a place to write to.
    82  	w, _ = w.ensure(1 + traceBytesPerNumber /* traceEvFrequency + frequency */)
    83  
    84  	// Write out the string.
    85  	w.byte(byte(traceEvFrequency))
    86  	w.varint(traceClockUnitsPerSecond())
    87  
    88  	// Immediately flush the buffer.
    89  	systemstack(func() {
    90  		lock(&trace.lock)
    91  		traceBufFlush(w.traceBuf, gen)
    92  		unlock(&trace.lock)
    93  	})
    94  }
    95
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