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runtime: invalid negative frequency while tracing

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The riscv64 Hifive Unmatched is the only platform that
failed on testcase TestAnalyzeAnnotations occasionally
after CL 332954 had merged. The failure happens when
ticks per second (freq) is over 1e12 which causing the timestamps
of two events are same.

There are 2 reasons causing big frequency:
1. RDCYCLE is HART based according to the riscv manual which makes
   negative ticks delta
2. negative float64 -> uint64 is undefined and "lucky" negative float
   is too big to handle for trace

For #46737

Change-Id: I1f3c1ac31aae249969000c719c32aaf5a66d29a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/373034
Trust: Zhuo Meng <mzh@golangcn.org>
Run-TryBot: Zhuo Meng <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
This commit is contained in:
Meng Zhuo 2021-12-17 14:15:42 +08:00 committed by Zhuo Meng
parent b357b05b70
commit a78532a412
2 changed files with 7 additions and 1 deletions
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5
src/runtime/asm_riscv64.s
View File

@ -81,7 +81,10 @@ TEXT setg_gcc<>(SB),NOSPLIT,$0-0
// func cputicks() int64
TEXT runtime·cputicks(SB),NOSPLIT,$0-8
RDCYCLE A0
// RDTIME to emulate cpu ticks
// RDCYCLE reads counter that is per HART(core) based
// according to the riscv manual, see issue 46737
RDTIME A0
MOV A0, ret+0(FP)
RET

3
src/runtime/trace.go
View File

@ -426,6 +426,9 @@ func ReadTrace() []byte {
trace.footerWritten = true
// Use float64 because (trace.ticksEnd - trace.ticksStart) * 1e9 can overflow int64.
freq := float64(trace.ticksEnd-trace.ticksStart) * 1e9 / float64(trace.timeEnd-trace.timeStart) / traceTickDiv
if freq <= 0 {
throw("trace: ReadTrace got invalid frequency")
}
trace.lockOwner = nil
unlock(&trace.lock)
var data []byte