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  	"internal/trace/tracev2"
    12  	_ "unsafe"
    13  )
    14  
    15  // Timestamps in trace are produced through either nanotime or cputicks
    16  // and divided by traceTimeDiv. nanotime is used everywhere except on
    17  // platforms where osHasLowResClock is true, because the system clock
    18  // isn't granular enough to get useful information out of a trace in
    19  // many cases.
    20  //
    21  // This makes absolute values of timestamp diffs smaller, and so they are
    22  // encoded in fewer bytes.
    23  //
    24  // The target resolution in all cases is 64 nanoseconds.
    25  // This is based on the fact that fundamentally the execution tracer won't emit
    26  // events more frequently than roughly every 200 ns or so, because that's roughly
    27  // how long it takes to call through the scheduler.
    28  // We could be more aggressive and bump this up to 128 ns while still getting
    29  // useful data, but the extra bit doesn't save us that much and the headroom is
    30  // nice to have.
    31  //
    32  // Hitting this target resolution is easy in the nanotime case: just pick a
    33  // division of 64. In the cputicks case it's a bit more complex.
    34  //
    35  // For x86, on a 3 GHz machine, we'd want to divide by 3*64 to hit our target.
    36  // To keep the division operation efficient, we round that up to 4*64, or 256.
    37  // Given what cputicks represents, we use this on all other platforms except
    38  // for PowerPC.
    39  // The suggested increment frequency for PowerPC's time base register is
    40  // 512 MHz according to Power ISA v2.07 section 6.2, so we use 32 on ppc64
    41  // and ppc64le.
    42  const traceTimeDiv = (1-osHasLowResClockInt)*64 + osHasLowResClockInt*(256-224*(goarch.IsPpc64|goarch.IsPpc64le))
    43  
    44  // traceTime represents a timestamp for the trace.
    45  type traceTime uint64
    46  
    47  // traceClockNow returns a monotonic timestamp. The clock this function gets
    48  // the timestamp from is specific to tracing, and shouldn't be mixed with other
    49  // clock sources.
    50  //
    51  // nosplit because it's called from exitsyscall and various trace writing functions,
    52  // which are nosplit.
    53  //
    54  // traceClockNow is called by golang.org/x/exp/trace using linkname.
    55  //
    56  //go:linkname traceClockNow
    57  //go:nosplit
    58  func traceClockNow() traceTime {
    59  	if osHasLowResClock {
    60  		return traceTime(cputicks() / traceTimeDiv)
    61  	}
    62  	return traceTime(nanotime() / traceTimeDiv)
    63  }
    64  
    65  // traceClockUnitsPerSecond estimates the number of trace clock units per
    66  // second that elapse.
    67  func traceClockUnitsPerSecond() uint64 {
    68  	if osHasLowResClock {
    69  		// We're using cputicks as our clock, so we need a real estimate.
    70  		return uint64(ticksPerSecond() / traceTimeDiv)
    71  	}
    72  	// Our clock is nanotime, so it's just the constant time division.
    73  	// (trace clock units / nanoseconds) * (1e9 nanoseconds / 1 second)
    74  	return uint64(1.0 / float64(traceTimeDiv) * 1e9)
    75  }
    76  
    77  // traceFrequency writes a batch with a single EvFrequency event.
    78  //
    79  // freq is the number of trace clock units per second.
    80  func traceFrequency(gen uintptr) {
    81  	w := unsafeTraceWriter(gen, nil)
    82  
    83  	// Ensure we have a place to write to.
    84  	w, _ = w.ensure(1 + traceBytesPerNumber /* tracev2.EvFrequency + frequency */)
    85  
    86  	// Write out the string.
    87  	w.byte(byte(tracev2.EvFrequency))
    88  	w.varint(traceClockUnitsPerSecond())
    89  
    90  	// Immediately flush the buffer.
    91  	systemstack(func() {
    92  		lock(&trace.lock)
    93  		traceBufFlush(w.traceBuf, gen)
    94  		unlock(&trace.lock)
    95  	})
    96  }
    97
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