Fix a few profiler related issues associated with frozen objects

[cshung]

This change fixes a few profiler-related issues associated with frozen objects.

1. Reset the pinned queue only once
2. Publish frozen objects
   3. Report the frozen segments as survived
3. Set frozen segment gen_num

Reset the pinned queue only once

This was an existing bug that is unrelated to frozen objects (for reason I don't understand it only got exposed recently).

In walk_relocation, we walk the generations (from old to young), the associated bricks (in region list order), and the associated plug trees (using an in-order traversal). During the walk, we dequeue the pinned plugs from the pinned plug queue.

There is only one pinned queue shared across all generations. Therefore we should not reset the pinned queue for every generation.

Publish frozen objects

Some profilers track object instances. If we don't report an object as allocated, they won't be tracking it, and they might be surprised if later on the object is reported as root.

Before the change, frozen objects are not reported as allocated.

After the change, frozen objects allocated through FrozenObjectHeapManager::TryAllocate are now reported to the profiler through PublishObjectAndNotify, and therefore they will be available through ICorProfiler::ObjectAllocated callback.

Report the frozen segments as survived

Some profilers might assume that objects previously allocated not falling in that survived range are being collected.

Before the change, frozen segments are not reported, and therefore the profiler might assume frozen objects are collected, while they are not, and then got surprised that they got reported as root.

After the change, the frozen segments are reported as survived, to do that, the walk_relocation_sip method is renamed to walk_relocation_special_segments to also handle the special case for frozen segments.

Set frozen segment gen_num

As we report the regions to the profiler, we will read the gen_num of the heap segment. The current code didn't set that value, so we might be reading random numbers in the profiler. That's not good. As an implementation detail, we thread the frozen segment into gen2, so we might as well set the gen_num to 2 there.

@dotnet/gc

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Issue Details

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This change fixes a few profiler-related issues associated with frozen objects.

1. Reset the pinned queue only once
2. Publish frozen objects
3. Report the frozen segments as survived
4. Set frozen segment gen_num

Reset the pinned queue only once

This was an existing bug that is unrelated to frozen objects (for reason I don't understand it only got exposed recently).

In walk_relocation, we walk the generations (from old to young), the associated bricks (in region list order), and the associated plug trees (using an in-order traversal). During the walk, we dequeue the pinned plugs from the pinned plug queue.

There is only one pinned queue shared across all generations. Therefore we should not reset the pinned queue for every generation.

Publish frozen objects

Some profilers track object instances. If we don't report an object as allocated, they won't be tracking it, and they might be surprised if later on the object is reported as root.

Before the change, frozen objects are not reported as allocated.

After the change, frozen objects allocated through FrozenObjectHeapManager::TryAllocate are now reported to the profiler through PublishObjectAndNotify, and therefore they will be available through ICorProfiler::ObjectAllocated callback.

Report the frozen segments as survived

Some profilers might assume that objects previously allocated not falling in that survived range are being collected.

Before the change, frozen segments are not reported, and therefore the profiler might assume frozen objects are collected, while they are not, and then got surprised that they got reported as root.

After the change, the frozen segments are reported as survived, to do that, the walk_relocation_sip method is renamed to walk_relocation_special_segments to also handle the special case for frozen segments.

Set frozen segment gen_num

As we report the regions to the profiler, we will read the gen_num of the heap segment. The current code didn't set that value, so we might be reading random numbers in the profiler. That's not good. As an implementation detail, we thread the frozen segment into gen2, so we might as well set the gen_num to 2 there.

@dotnet/gc

Author:	cshung
Assignees:	cshung
Labels:	area-Diagnostics-coreclr
Milestone:	-

[cshung]

dotnet/diagnostics#4156

[PeterSolMS]

Looks good to me!

[davmason]

LGTM from the profiler side, I am not a GC expert though :)

[cshung]

@Maoni0 and I had a discussion on this and we believe reporting the frozen segments as a plug is inappropriate. We need a different fix for that part.