runtime: add GODEBUG=dontfreezetheworld=1

This GODEBUG flag disables the freezetheworld call during fatal panic.
freezetheworld asks the scheduler to stop running goroutines on all Ms.
This is normally useful, as it ensures we can collect a traceback from
every goroutine. However, it can be frustrating when debugging the
scheduler itself, as it significantly changes the scheduler state from
when the panic started.

Setting this flag has some disadvantages. Most notably, running
goroutines will not traceback in the standard output (though they may be
included in the final SIGQUIT loop). Additionally, we may missing
concurrently created goroutines when looping over allgs (CL 270861 made
this safe, but still racy). The final state of all goroutines will also
be further removed from the time of panic, as they continued to run for
a while.

One unfortunate part of this flag is the final SIGQUIT loop in the
runtime leaves every thread in the signal handler at exit. This is a bit
frustrating in gdb, which doesn't understand how to step beyond
sigtramp. The data is still there, but you must manually walk.

Change-Id: Ie6bd3ac521fcababea668196b60cf225a0be1a00
Reviewed-on: https://go-review.googlesource.com/c/go/+/478975
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

diff --git a/src/runtime/extern.go b/src/runtime/extern.go
index 55dfbff7c40d7b1ea4a45ba3e0793c1680d51e5c..8b92108c705273d3f745b68b80f690e1531b37f6 100644 (file)
--- a/src/runtime/extern.go
+++ b/src/runtime/extern.go
@@ -55,6 +55,15 @@ It is a comma-separated list of name=val pairs setting these named variables:
        cgocheck mode can be enabled using GOEXPERIMENT (which
        requires a rebuild), see https://pkg.go.dev/internal/goexperiment for details.
 
+       dontfreezetheworld: by default, the start of a fatal panic or throw
+       "freezes the world", stopping all goroutines, which makes it possible
+       to traceback all goroutines (running goroutines cannot be traced), and
+       keeps their state close to the point of panic. Setting
+       dontfreezetheworld=1 disables freeze, allowing goroutines to continue
+       executing during panic processing. This can be useful when debugging
+       the runtime scheduler, as freezetheworld perturbs scheduler state and
+       thus may hide problems.
+
        efence: setting efence=1 causes the allocator to run in a mode
        where each object is allocated on a unique page and addresses are
        never recycled.

diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index ccc464371175fb9a780a4d418e2d4caa51241c65..89070d226cfca7176ee85509c59d1fbf645a2d79 100644 (file)
--- a/src/runtime/panic.go
+++ b/src/runtime/panic.go
@@ -1242,6 +1242,9 @@ func startpanic_m() bool {
                if debug.schedtrace > 0 || debug.scheddetail > 0 {
                        schedtrace(true)
                }
+               if debug.dontfreezetheworld > 0 {
+                       return true
+               }
                freezetheworld()
                return true
        case 1:

diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index 991b92a0af6ec0b9e487b181802ff92f5278e0e1..02237685c794fefc6a46809d4e7192c37a084833 100644 (file)
--- a/src/runtime/runtime1.go
+++ b/src/runtime/runtime1.go
@@ -309,6 +309,7 @@ type dbgVar struct {
 var debug struct {
        cgocheck           int32
        clobberfree        int32
+       dontfreezetheworld int32
        efence             int32
        gccheckmark        int32
        gcpacertrace       int32
@@ -340,6 +341,7 @@ var dbgvars = []*dbgVar{
        {name: "allocfreetrace", value: &debug.allocfreetrace},
        {name: "clobberfree", value: &debug.clobberfree},
        {name: "cgocheck", value: &debug.cgocheck},
+       {name: "dontfreezetheworld", value: &debug.dontfreezetheworld},
        {name: "efence", value: &debug.efence},
        {name: "gccheckmark", value: &debug.gccheckmark},
        {name: "gcpacertrace", value: &debug.gcpacertrace},