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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jan 31, 2022, at 4:19 PM, Florian Weimer fw@deneb.enyo.de wrote:

> * 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).

I suspect that any approach based on a user-space cache will break with
respect to CRIU. That is one big advantage of using the rseq thread area
for this.

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

As stated in my self-reply, I don't think those two fields are needed
after all.

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

True. However considering that we plan to remove the buggy "rseq_cs.ptr"
fields from the API, that rseq.h UAPI compatibility does not seem to be
very much relevant.

But still, it's better if we can avoid breaking API, agreed. And the
"node_id_start" does not appear to be needed after all.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com