linux-kernel - Re: [PATCH 1/1] rseq.2: New man page for the rseq(2) API

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Date:   Wed, 15 Feb 2023 12:12:52 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     "G. Branden Robinson" <g.branden.robinson@...il.com>,
        Alejandro Colomar <alx.manpages@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH 1/1] rseq.2: New man page for the rseq(2) API

On 2023-02-15 12:09, Mathieu Desnoyers wrote:
> On 2023-02-14 20:20, G. Branden Robinson wrote:
[...]
>>
>>>> +user-space performs any side-effect
>>>> +(e.g. storing to memory).
>>>> +.IP
>>>> +This field is always guaranteed to hold a valid CPU number in the 
>>>> range
>>>> +[ 0 ..  nr_possible_cpus - 1 ].
>>>
>>> Please use interval notation:
>>>     [0, nr_possible_cpus)
>>> or
>>>     [0, nr_possible_cpus - 1]
>>> whichever looks better to you.
>>>
>>> We did some consistency fix recently:
>>> <https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=147a60d792a5db8f3cb93ea16eefb73e16c1fb91>
>>>
>>> Also, do we have a more standard way of saying nr_possible_cpus?
>>> Should we say nproc?
> 
> nproc(1) means:
> 
>         Print  the number of processing units available to the current
>         process, which may be less than the number of online processors
> 
> Which is the number of cpus currently available (AFAIU the result of the
> cpuset and sched affinity).
> 
> What I really mean here is the maximum value for possible cpus which can 
> be hotplugged into the system. So it's not the maximum number of 
> possible CPUs per se, but rather the maximum enabled bit in the possible 
> CPUs mask.
> 
> Note that we could express this differently as well: rather than saying 
> that it guarantees a value in the range [0, nr_possible_cpus - 1], we 
> could say that the values are guaranteed to be part of the possible cpus 
> mask, which would actually more accurate in case the possible cpus mask 
> has a hole (it tends to happen with things like lxc containers nowadays).
> 
> Do you agree that we should favor expressing this in terms of belonging 
> to the possible cpumask set rather than a range starting from 0 ?

Actually, the field may contain the value 0 even if 0 is not part of the 
possible cpumask. So forget what I just said about being guaranteed to 
be part of the possible cpus mask.

Thoughts ?

Thanks,

Mathieu

> 
> Thanks,
> 
> Mathieu
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com