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Date: Sat, 07 Aug 2021 18:29:56 +0200
From: Leah Neukirchen <leah@...u.org>
To: Vincent Donnefort <vincent.donnefort@....com>
Cc: Rich Felker <dalias@...c.org>, musl@...ts.openwall.com, Samuel Holland
<samuel@...lland.org>, jason <jason@...omnia247.nl>
Subject: Re: [PATCH v3] sysconf: add _SC_NPROCESSORS_CONF support
Vincent Donnefort <vincent.donnefort@....com> writes:
>> > Moreover, in the case of a non-available sysfs, the fallback to sched_getaffinity
>> > would help and at least align _CONF with _ONL (which is the current behavior).
>> >
>> > Reading /proc/stat and /proc/softirq looks like a hack. They just happen to
>> > align on the _ONL/_CONF and doesn't produce a machine-readable output. While
>> > the CPU sysfs subsys is really meant to describe CPUs.
>>
>> If they're mandated stable interfaces that "happen" to have the exact
>> data we need, I don't see what the objection to using them is. A
>> better question would be whether access to them is restricted in any
>> hardening profiles. At least they don't seem to be hidden by the
>> hidepid mount option.
>>
> Indeed the function hasn't changed for 10y, which is I guess somewhat stable
> and it is currently walking through all the possible CPUs (which is what we want
> to count). Also, this file is currently always present whenever procfs is
> mounted.
>
> Nonetheless, as this is human-readable, nothing mandates the format. And as an
> example, on my desktop machine, with 448 allocated CPUS, the first line of
> /proc/softirqs (the line that contains all the CPUs) is 4949 long. The
> "possible" mask describes same information with only 6 characters.
For the record, on my single-cpu mainboard with a "AMD Ryzen 7 3700X
8-Core Processor", there are 16 processor entries in /proc/cpuinfo,
but /proc/softirq goes to CPU31. /sys/devices/system/cpu has entries
up to cpu15, and glibc getconf _NPROCESSORS_CONF also says 16.
--
Leah Neukirchen <leah@...u.org> https://leahneukirchen.org/
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