If we add a try bot, it would be only one platform, like linux_layout_tests_slimming_paint_v2.

Thanks to skobes@, you can now also do try jobs with this flag by including this flag in the file LayoutTests/rwt.flag in the CL that is being tried. Then you can trigger all of the *_blink_rel try bots or {linux,mac,win}_chromium_rel_ng bots.

Would this way be convenient enough?

Yeah, just on linux is fine for now as this area is primarily in layout and does not have large platform differences. I think it makes sense to re-use the existing slimming paint v2 pool of bots.

I've been following Steve's work on the flag try jobs but I think we still want a dedicated bot for two reasons:
1) A dedicated trybot is useful for ensuring root layer scrolling patches pass tests when committing via the additional cq bots flag. In other words, it makes root layer scrolling commits atomic.
2) As we get closer to launching root layer scrolling, we will need a real CQ bot to ensure other teams do not break root layer scrolling codepaths. I think we'll gradually add presubmit scripts just like we've done for slimming paint v2.

I also think the dedicated bot approach is simpler, albeit there is a one-time infra setup cost. The paint team has been more productive with the spv2 trybot approach and I think the same benefits can be brought to the layout team.

We now have <400 tests remaining (aka <400 out of 63783 fail) and are getting closer to our OKR of <100 passing, and to launching. I think it would really help our productivity to have a linux-only bot at this time. Would it be possible to increase the priority of this bug?

This actually shouldn't be blocked on anything if we reuse VMs from an existing pool. I'll upload CLs to change the configs to add this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/325b25e0a0c8fc55907b697f75d5d9453b52bc00

commit 325b25e0a0c8fc55907b697f75d5d9453b52bc00
Author: Quinten Yearsley <qyearsley@chromium.org>
Date: Sat Oct 28 01:52:01 2017

Add root layer scroll optional trybot (src-side)

Bug:  771753 
Change-Id: I52573b3d8eac920ec8a89ef46be220c40042768d
Reviewed-on: https://chromium-review.googlesource.com/734263
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512370}
[modify] https://crrev.com/325b25e0a0c8fc55907b697f75d5d9453b52bc00/testing/buildbot/chromium.fyi.json
[modify] https://crrev.com/325b25e0a0c8fc55907b697f75d5d9453b52bc00/tools/mb/mb_config.pyl

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/e4767c6b87a487c2c5c932332d61d72a2ccfadf9

commit e4767c6b87a487c2c5c932332d61d72a2ccfadf9
Author: Quinten Yearsley <qyearsley@chromium.org>
Date: Mon Oct 30 16:48:58 2017

Add root layer scroll optional trybot (build-side)

Bug:  771753 
Change-Id: I4bc731d1d9b64f626c5da4551fa061e6f09eb6c9
Reviewed-on: https://chromium-review.googlesource.com/734212
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

[modify] https://crrev.com/e4767c6b87a487c2c5c932332d61d72a2ccfadf9/scripts/slave/recipe_modules/chromium_tests/trybots.py
[modify] https://crrev.com/e4767c6b87a487c2c5c932332d61d72a2ccfadf9/masters/master.tryserver.chromium.linux/slaves.cfg
[modify] https://crrev.com/e4767c6b87a487c2c5c932332d61d72a2ccfadf9/scripts/slave/recipe_modules/chromium_tests/chromium_fyi.py
[modify] https://crrev.com/e4767c6b87a487c2c5c932332d61d72a2ccfadf9/masters/master.tryserver.chromium.linux/master.cfg

The bot should now be present: https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_layout_tests_root_layer_scrolls

I think it won't appear on the "Choose Trybots" UI in Gerrit until some config is updated, but you should be able to use:

    git cl try -b linux_layout_tests_root_layer_scrolls -m tryserver.chromium.linux

Does that work?

I did a simple test with Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_root_layer_scrolls but it failed with:

Uncaught Exception: AssertionError('No bot config for master/builder [tryserver.chromium.linux / linux_layout_tests_root_layer_scrolls]',)

https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_layout_tests_root_layer_scrolls/builds/2
https://chromium-review.googlesource.com/c/chromium/src/+/745323

This might need time to propagate. I will try again tomorrow and report back.

I tried this again this morning and got the same failure:

Uncaught Exception: AssertionError('No bot config for master/builder [tryserver.chromium.linux / linux_layout_tests_root_layer_scrolls]',)

https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_layout_tests_root_layer_scrolls/builds/3

https://chromium-review.googlesource.com/c/chromium/src/+/746964

This time I used a fresh patch and the git cl command line "git cl try -b linux_layout_tests_root_layer_scrolls -m tryserver.chromium.linux"

I think I found the mistake -- I didn't add the name right here: https://chromium-review.googlesource.com/c/chromium/tools/build/+/734212/4/scripts/slave/recipe_modules/chromium_tests/trybots.py#426
Now uploading a fix.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/1e135db3636f755e6b755425c7faa81e62411879

commit 1e135db3636f755e6b755425c7faa81e62411879
Author: Quinten Yearsley <qyearsley@chromium.org>
Date: Tue Oct 31 16:35:26 2017

Fix name of new root_layer_scrolls bot in chromium_tests config

This is a follow-up fix to:
https://chromium-review.googlesource.com/c/chromium/tools/build/+/734212

TBR: dpranke
Bug:  771753 
Change-Id: Ib9fe3b10e1d446b0208d04ce00dbc18a32cc0dea
Reviewed-on: https://chromium-review.googlesource.com/746982
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>

[modify] https://crrev.com/1e135db3636f755e6b755425c7faa81e62411879/scripts/slave/recipe_modules/chromium_tests/trybots.py

Woohoo!
https://chromium-review.googlesource.com/c/chromium/src/+/748910

Thank you Quinten!