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From:   Florian Weimer <fw@deneb.enyo.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org,
        Christian Brauner <christian.brauner@ubuntu.com>,
        David.Laight@ACULAB.COM, carlos@redhat.com,
        Peter Oskolkov <posk@posk.io>
Subject: Re: [RFC PATCH 1/2] rseq: extend struct rseq with numa node id
References: <20220131205531.17873-1-mathieu.desnoyers@efficios.com>
Date:   Mon, 31 Jan 2022 22:19:09 +0100
In-Reply-To: <20220131205531.17873-1-mathieu.desnoyers@efficios.com> (Mathieu
        Desnoyers's message of "Mon, 31 Jan 2022 15:55:30 -0500")
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* 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.