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

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Date:   Mon, 31 Jan 2022 16:26:06 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api <linux-api@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        David Laight <David.Laight@...LAB.COM>,
        carlos <carlos@...hat.com>, Peter Oskolkov <posk@...k.io>
Subject: Re: [RFC PATCH 1/2] rseq: extend struct rseq with numa node id

----- On Jan 31, 2022, at 4:19 PM, Florian Weimer fw@...eb.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