rt: make the LIFO slot in the multi-threaded scheduler stealable

[carllerche]

The multi-threaded scheduler includes a per-worker LIFO slot used to store the last scheduled task. This can improve certain usage patterns, especially message passing between tasks. However, this LIFO slot is not currently stealable.

As a temporary workaround, #4936 adds an unstable configuration option to disable the LIFO slot.

[MortenLohne]

Is there any further information available on this? I seem to have hit some pathological worst-case behavior with very cpu-heavy tasks, where every thread except one ended up totally stalled. Disabling the LIFO slot resolved the problem, giving me a 20x speedup.

[Darksonn]

It has not been implemented yet.

[rcoh]

Should we make a counter that tracks times when if possible, the lifo slot would have been stolen? This could help application authors know if they should try the mitigation.

[gengfanbin]

thank