cost for interruption in an rseq critical section is that the PC gets overwritten to the rseq abort entry point before the task is rescheduled. no management thread necessary.
should be fairly minimal cost, especially assuming interruptions in the critical section are rare.
Giving it some more thought, I expect caches will typically be wiped out by a context switch. So the only place rseq is likely to benefit an allocator is on systems with multiple NUMA nodes where you'd like to make sure any management code isn't paying a penalty by hitting the wrong address range.
IIUC rseq (ie CPU local data) is primarily good for two things. The first being obviating the need for atomics (specifically the resultant cache line ping-pong) but thread local data already accomplishes that. The second being massive oversubscription of physical CPU cores (ie tens of thousands of threads) where TLS becomes utterly wasteful while also thrashing the cache.
maybe i give it too much weight, but:
> massive oversubscription of physical CPU cores (ie tens of thousands of threads) where TLS becomes utterly wasteful while also thrashing the cache
seems worth solving to me