Full disclosure, I work on LTTng and a 20% improvement is a huge deal to our users, so I'm certainly biased here.

As far as I know, the thread-local write only occurs if a rseq region is set. The added cost to a context switch seems essentially limited to a branch: https://github.com/torvalds/linux/commit/d7822b1e24f2df5df98...

The other benchmarks that were posted in the various threads leading to this merge looked fairly impressive to me.

Of the top of my head, jemalloc benefited a lot from this patchset, and not just on the run-time front: https://lkml.org/lkml/2016/10/10/332