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From: Vincent Donnefort <vincent.donnefort@arm.com>
To: jyknight@google.com, musl@lists.openwall.com
Subject: Re: [musl] Re: Re: [PATCH v2] sysconf: add _SC_NPROCESSORS_CONF support
Date: Tue, 6 Jul 2021 10:00:20 +0100	[thread overview]
Message-ID: <20210706090020.GB216826@e120877-lin.cambridge.arm.com> (raw)
In-Reply-To: <20210703212018.GA3554268@port70.net>

On Sat, Jul 03, 2021 at 11:20:18PM +0200, Szabolcs Nagy wrote:
> * Vincent Donnefort <vincent.donnefort@arm.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

      reply	other threads:[~2021-07-06  9:00 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-02 13:29 Vincent Donnefort
2021-07-03 21:20 ` Szabolcs Nagy
2021-07-06  9:00   ` Vincent Donnefort [this message]

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