Thanks for taking this up. The split based on tree priority sounds great. It would also be nice for the PendingTreeDuration UMAs to be split based on impl-side vs main-side pending trees.

We already have an UMA for commit initiated pending trees -
 CommitToReadyToActivateDuration. I think what you want is an InvalidateToReadyToActivateDuration UMA?

fwiw I think PendingTreeDuration is a better name than CommitToReadyToActivateDuration so I might steal it.

I think CommitToReadyToActivateDuration would be closer to PendingTreeRasterDuration. PendingTreeDuration also includes the commit + wait for activation time. So may be PendingTreeRasterDuration.Main/Impl?

Do we have any UMA that captures the time the pending tree waits for activation after ready to activate?

We also have CommitToActivateDuration which captures the time spent during activation too (which PendingTreeDuration does not).

So...The tree priority can keep changing while we are working on a pending tree. How do we decide which bucket to put it in? :P

Maybe we another bucket for "multiple priorities oops".

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ca2c1cf2d9748d25050bfe7f71ee40463588a6a9

commit ca2c1cf2d9748d25050bfe7f71ee40463588a6a9
Author: Khushal <khushalsagar@chromium.org>
Date: Thu Jul 27 23:06:43 2017

cc: Split pending tree duration metrics based on main/impl side tree.

Add a metric to measure the duration to prepare the pending tree
initiated from impl-side invalidations, analogous to the commit to
ready to activate duration, and a metric to measure the
wait time for a ready to activate tree split based on main or impl side
tree.

Bug:  738151 
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I5201519a5510d238f6efab36493d7ce17e751636
Reviewed-on: https://chromium-review.googlesource.com/566184
Reviewed-by: Brian Anderson <brianderson@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490063}
[modify] https://crrev.com/ca2c1cf2d9748d25050bfe7f71ee40463588a6a9/cc/scheduler/compositor_timing_history.cc
[modify] https://crrev.com/ca2c1cf2d9748d25050bfe7f71ee40463588a6a9/cc/scheduler/compositor_timing_history.h
[modify] https://crrev.com/ca2c1cf2d9748d25050bfe7f71ee40463588a6a9/cc/scheduler/compositor_timing_history_unittest.cc
[modify] https://crrev.com/ca2c1cf2d9748d25050bfe7f71ee40463588a6a9/cc/scheduler/scheduler.cc
[modify] https://crrev.com/ca2c1cf2d9748d25050bfe7f71ee40463588a6a9/cc/scheduler/scheduler_state_machine.cc
[modify] https://crrev.com/ca2c1cf2d9748d25050bfe7f71ee40463588a6a9/cc/scheduler/scheduler_state_machine.h
[modify] https://crrev.com/ca2c1cf2d9748d25050bfe7f71ee40463588a6a9/tools/metrics/histograms/histograms.xml