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

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Message-ID: <Y2pT7ij/TcI4EmH6@hirez.programming.kicks-ass.net>
Date:   Tue, 8 Nov 2022 14:04:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org, Christian Brauner <brauner@...nel.org>,
        Florian Weimer <fw@...eb.enyo.de>, David.Laight@...lab.com,
        carlos@...hat.com, Peter Oskolkov <posk@...k.io>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Chris Kennelly <ckennelly@...gle.com>
Subject: Re: [PATCH v5 08/24] sched: Introduce per memory space current
 virtual cpu id

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.