All graphs for this bug:
  https://chromeperf.appspot.com/group_report?bug_id=834500

(For debugging:) Original alerts at time of bug-filing:
  https://chromeperf.appspot.com/group_report?sid=1cbea890b1d53749225a43718ae15a7771d8a60ec876517ea54d61aa30fbc788


Bot(s) for this bug's original alert(s):

cros-hana

Pretty bad perf regression, I'm bisecting...

a96f5f8168db928f0ac94ee9a4c4f32a6dc8444f is the first bad commit
commit a96f5f8168db928f0ac94ee9a4c4f32a6dc8444f
Author: Kenneth Russell <kbr@chromium.org>
Date:   Fri Apr 6 04:22:02 2018 +0000

    Make clearBuffer APIs work with preserveDrawingBuffer:false.
    
    The state of which buffers (color, depth, stencil) were modified was
    not being tracked properly with the introduction of the ES 3.0 / WebGL
    2.0 APIs. Generalize DrawingBuffer's tracking, and make these APIs
    update the state properly.
    
    Tested with new WebGL conformance test in
    https://github.com/KhronosGroup/WebGL/pull/2628 .
    
    Bug:  828262 
    Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2;master.tryserver.chromium.win:win_optional_gpu_tests_rel
    Change-Id: I8d87b1940a73c689c2107d969139ad8c9c265ce5
    Reviewed-on: https://chromium-review.googlesource.com/994463
    Commit-Queue: Kenneth Russell <kbr@chromium.org>
    Reviewed-by: Kai Ninomiya <kainino@chromium.org>
    Reviewed-by: Zhenyao Mo <zmo@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#548667}

:040000 040000 b91882ce2adfc7664105ad2a8e5861316ef084b9 a40356927ef5daca83ee8df4f1c738372d765200 M      third_party


Ken can you take a look?

Argh, it sounds like I accidentally introduced a double-clear or something similar. I'm sorry.

How can I easily reproduce this myself? Or can I sit down with you to debug it?

You can come over any time. You need a mediatek-based device I *think*.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b542b9764af4418d4db62439fa43d006224ab1c1

commit b542b9764af4418d4db62439fa43d006224ab1c1
Author: Kenneth Russell <kbr@chromium.org>
Date: Fri Apr 20 02:25:24 2018

Fix clearing of implicit stencil buffer.

This was lost in the recent DrawingBuffer refactoring adding
clearBuffer support, leading to performance losses on lower-end GPUs.

Bug:  834500 
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ic81ac215183419145e7e9a67552e760cd3854764
Reviewed-on: https://chromium-review.googlesource.com/1020527
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552234}
[modify] https://crrev.com/b542b9764af4418d4db62439fa43d006224ab1c1/third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc

Requesting merge back to M67. Otherwise WebGL on some ChromeOS devices and probably older Android devices will get slower.

Unrestricting view.

Approving merge to M67 Chrome OS.

Merged to M67 in:
https://chromium-review.googlesource.com/1023337
https://chromium.googlesource.com/chromium/src/+/97ed5877ea0d702a4c2a80a38096d585838d0206

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/97ed5877ea0d702a4c2a80a38096d585838d0206

commit 97ed5877ea0d702a4c2a80a38096d585838d0206
Author: Kenneth Russell <kbr@chromium.org>
Date: Sat Apr 21 19:21:52 2018

Fix clearing of implicit stencil buffer.

This was lost in the recent DrawingBuffer refactoring adding
clearBuffer support, leading to performance losses on lower-end GPUs.

TBR=kbr@chromium.org

(cherry picked from commit b542b9764af4418d4db62439fa43d006224ab1c1)

Bug:  834500 
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ic81ac215183419145e7e9a67552e760cd3854764
Reviewed-on: https://chromium-review.googlesource.com/1020527
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#552234}
Reviewed-on: https://chromium-review.googlesource.com/1023337
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/branch-heads/3396@{#194}
Cr-Branched-From: 9ef2aa869bc7bc0c089e255d698cca6e47d6b038-refs/heads/master@{#550428}
[modify] https://crrev.com/97ed5877ea0d702a4c2a80a38096d585838d0206/third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc

The fishes are rejoicing on the branch and on master. Thank you!