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

[Florian Weimer <fw@deneb.enyo.de>]

* 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.