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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