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

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Message-ID: <87bkzq23cl.fsf@mid.deneb.enyo.de>
Date:   Tue, 01 Feb 2022 13:11:54 +0100
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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 <christian.brauner@...ntu.com>,
        David.Laight@...LAB.COM, carlos@...hat.com,
        Peter Oskolkov <posk@...k.io>
Subject: Re: [RFC PATCH 1/2] rseq: extend struct rseq with numa node id

* Peter Zijlstra:

> On Mon, Jan 31, 2022 at 10:19:09PM +0100, Florian Weimer wrote:
>> 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).
>
> PowerPC does that quite a lot..

How so?  Why make it especially hard for userspace?