Re: [PATCH v5 08/24] sched: Introduce per memory space current virtual cpu id

[Peter Zijlstra]

On Thu, Nov 03, 2022 at 04:03:43PM -0400, Mathieu Desnoyers wrote:

> The credit goes to Paul Turner (Google) for the vcpu_id idea. This
> feature is implemented based on the discussions with Paul Turner and
> Peter Oskolkov (Google), but I took the liberty to implement scheduler
> fast-path optimizations and my own NUMA-awareness scheme. The rumor has
> it that Google have been running a rseq vcpu_id extension internally at
> Google in production for a year. The tcmalloc source code indeed has
> comments hinting at a vcpu_id prototype extension to the rseq system
> call [1].

Re NUMA thing -- that means that on a 512 node system a single threaded
task can still observe 512 separate vcpu-ids, right?

Also, said space won't be dense.

The main selling point of the whole vcpu-id scheme was that the id space
is dense and not larger than min(nr_cpus, nr_threads), which then gives
useful properties.

But I'm not at all seeing how the NUMA thing preserves that.

Also; given the utter mind-bendiness of the NUMA thing; should it go
into it's own patch; introduce the regular plain old vcpu first, and
then add things to it -- that also allows pushing those weird cpumask
ops you've created later into the series.