Fix issue 41311 - Assert failure ephemeral_heap_segment->saved_committed == heap_segment_committed

[PeterSolMS]

This is an interaction problem between gradual decommit and entering a no GC region.

Details:

• When we enter a no GC region, we are manipulating our commit goals outside of GC, we usually are planning to commit more, so it makes sense to stop any gradual decommit in progress.
• Even if we need to do a GC in preparation for a no GC region, any decommit target established by decommit_ephemeral_segment_pages is likely to be invalid, so we do an early out.
• add an assert to grow_heap_segment so we can catch future issues in this area better.

We may want to trigger the gradual decommit logic in set_allocations_for_no_gc if we discover that after accounting for the planned allocation, we still have substantial decommits. I'm leaving this for a later PR

cc @xiangzhai

[ghost]

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[Maoni0]

I must be missing something - isn't decommit_target smaller than committed? we keep shrinking committed till we reach decommit_tagert when we do the decommit steps.

also when gradual_decommit_in_progress_p is TRUE, shouldn't decommit_target always be non NULL?

nit -

        assert (!gradual_decommit_in_progress_p ||
                 (heap_segment_decommit_target (seg) == nullptr) ||
                 (heap_segment_committed (seg) <= heap_segment_decommit_target (seg)));

[PeterSolMS]

For some heaps, decommit_target may be bigger than committed, and that's fine - it just means that we won't decommit.

When gradual_decommit_in_progress_p is TRUE, decommit_target will be non NULL for the ephemeral_heap_segment, but may be NULL for other segments - the condition was meant to filter out the other segments. Now that I think about it, I realize this isn't a good test - I should compare against ephemeral_heap_segment directly. So I fixed the assert as follows:

    // we should never increase committed beyond decommit target when gradual
    // decommit is in progress - if we do, this means commit and decommit are
    // going on at the same time.
    assert (!gradual_decommit_in_progress_p ||
            (seg != ephemeral_heap_segment) ||
            (heap_segment_committed (seg) <= heap_segment_decommit_target (seg)));

[Maoni0]

oh right, I was only thinking about eph seg. and yes the point is if we actually need to commit more, the decommit target had better be bigger otherwise we have a problem. thanks! I agree the new assert looks better.

[Maoni0]

if (settings.concurrent || use_large_pages_p || settings.pause_mode == pause_no_gc) [](start = 4, length = 83)

nit

if (settings.concurrent || use_large_pages_p || (settings.pause_mode == pause_no_gc))

[PeterSolMS]

/backport to release/5.0

[github-actions]

Started backporting to release/5.0: https://github.com/dotnet/runtime/actions/runs/234190938