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Message-ID: <20210703212018.GA3554268@port70.net>
Date: Sat, 3 Jul 2021 23:20:18 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Vincent Donnefort <vincent.donnefort@....com>
Cc: jyknight@...gle.com, musl@...ts.openwall.com
Subject: Re: Re: Re: [PATCH v2] sysconf: add _SC_NPROCESSORS_CONF
support
* Vincent Donnefort <vincent.donnefort@....com> [2021-07-02 14:29:37 +0100]:
> Here's a new thread as I never received your previous email and I just
> noticed the answer today.
>
> I'm not sure I understand why I can't use "present". As per the kernel
> documentation:
>
> possible: CPUs that have been allocated resources and can be
> brought online if they are present. [cpu_possible_mask]
>
> present: CPUs that have been identified as being present in the
> system. [cpu_present_mask]
>
>
> In this example, the NR_CPUS config option is 128, but the kernel was
> started with possible_cpus=144. There are 4 CPUs in the system and cpu2
> was manually taken offline (and is the only CPU that can be brought
> online.)::
>
> kernel_max: 127
> offline: 2,4-127,128-143
> online: 0-1,3
> possible: 0-127
> present: 0-3
>
> So indeed I could use "possible"... but there's a chance the two masks won't
> be equal, and the sysfs entries are matching "present", not "possible".
>
> "possible" is the CPUs that have allocated resources and can be physically
> added to the system. "present" is the CPUs that are known as physically
> present but might be offline.
i guess we just need a guarantee that these interfaces are stable
and the set of present cpus don't change during the lifetime of a
process. (can that even work with checkpoint/restore? probably not
our problem)
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