tree b202073980d8eb1c4dd3f791adddc6fdfc7048dd
parent bb340ea8a4cadf31f9039fe1d2b21dd7f8121770
author jamesr <jamesr@chromium.org> 1477343772 -0700
committer Commit bot <commit-bot@chromium.org> 1477343876 +0000

[gn] Add trace entries for loading and blocking on imports

This adds trace entries for loading imports and for blocking on an
import for longer than 20ms. These make gn's tracelog significantly
easier to understand when imported files take a long time to process.
For example, in the Chrome build on Mac //build/toolchain/mac/BUILD.gn
blocks for ~240ms running sdk_info.py because
//build/config/mac/mac_sdk.gni runs that script and //ui/base/BUILD.gn
blocks for ~350ms because it is waiting for the import lock on
mac_sdk.gni to be released. Without these traces, it is not
immediately obvious why //build/toolchain/mac/BUILD.gn is running that
script and it is very unclear why //ui/base/BUILD.gn takes so long.

Review-Url: https://codereview.chromium.org/2424233002
Cr-Original-Commit-Position: refs/heads/master@{#427151}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 840ab52b0bbdbe46a093c76c4a57b505d7e29e17