1818243 - Investigate hangs in CycleCollectedJSContext::PerformMicroTaskCheckPoint reported by BHR

Status: NEW

(Core :: Cycle Collector, task)

Description

[Jens Stutte [:jstutte]]

It seems we can get stuck in CycleCollectedJSContext::PerformMicroTaskCheckPoint.

In particular there are stacks that indicate we are hanging directly inside that function, not within something called from it:

mozilla::CycleCollectedJSContext::PerformMicroTaskCheckPoint(bool) xul

which would probably mean that we are hanging directly inside its for loop.

Note about BHR: It truncates stacks up to the first event loop processing and if there is a bhr: whiteboard tag, it will group and truncate stacks also down to the first line that contains the search string. But those single line stacks were already present before adding the bhr: whiteboard tag here, so I must assume they are real.

Comment 1

[Jens Stutte [:jstutte]]

Adding the BHR annotation actually groups more hangs than expected. Not all of them originally read just with one frame, but there is a decent amount of them that apparently hang directly in the for loop inside PerformMicroTaskCheckPoint. The latest optimization around mSuppressedMicroTasks might be of interest here ?

Comment 2

[Olli Pettay [:smaug][[e-mail]]]

It is somewhat expected to see hangs under PerformMicroTaskCheckPoint. That is where long js promise chains are handled.
Do you have a link to something where mSuppressedMicroTasks shows up?

Comment 3

[Jens Stutte [:jstutte]]

(In reply to Olli Pettay [:smaug][[e-mail]] from comment #2)

It is somewhat expected to see hangs under PerformMicroTaskCheckPoint. That is where long js promise chains are handled.
Do you have a link to something where mSuppressedMicroTasks shows up?

Sorry, the description omitted some context. See updated description.

Comment 4

[Jens Stutte [:jstutte]]

Removing [bhr:PerformMicroTaskCheckPoint] to check the full stacks.

Comment 5

[Jens Stutte [:jstutte]]

So the empty stack this bug is interested in ranks #17 currently.

It is probably better for other bugs (like bug 1688963) to not add the whiteboard bhr tag here.

Comment 6

[Olli Pettay [:smaug][[e-mail]]]

I guess the hang could look like this if there are tons of promises