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Message-ID: <20210706090020.GB216826@e120877-lin.cambridge.arm.com>
Date: Tue, 6 Jul 2021 10:00:20 +0100
From: Vincent Donnefort <vincent.donnefort@....com>
To: jyknight@...gle.com, musl@...ts.openwall.com
Subject: Re: Re: Re: [PATCH v2] sysconf: add _SC_NPROCESSORS_CONF
support
On Sat, Jul 03, 2021 at 11:20:18PM +0200, Szabolcs Nagy wrote:
> * 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)
In that case "possible" is what we want. It can't change during the kernel
lifetime, while "present" can.
--
Vincent
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