Re: [RFC PATCH 1/2] rseq: extend struct rseq with numa node id

* Mathieu Desnoyers:

Adding the NUMA node id to struct rseq is a straightforward thing to do,
and a good way to figure out if anything in the user-space ecosystem
prevents extending struct rseq.

This NUMA node id field allows memory allocators such as tcmalloc to
take advantage of fast access to the current NUMA node id to perform
NUMA-aware memory allocation.

It is also useful for implementing NUMA-aware user-space mutexes.

It can be used to implement getcpu purely in userspace, too.  I had
plan to hack this together with a node ID cache in TLS, which should
offer pretty much the same functionality (except for weird CPU
topology changes which alter the node ID of a previously used CPU).

However, I do not understand the need for two fields here.  Why isn't
one enough?

One field would also avoid the need to mess with rseq_cpu_id_state,
maintaining API compatibility.