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Date: Tue, 2 Apr 2019 21:02:53 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: musl <musl@...ts.openwall.com>, Michael Jeanson <mjeanson@...icios.com>,
	Richard Purdie <richard.purdie@...uxfoundation.org>,
	Jonathan Rajotte <jonathan.rajotte-julien@...icios.com>
Subject: Re: sysconf(_SC_NPROCESSORS_CONF) returns the wrong value

* Mathieu Desnoyers <mathieu.desnoyers@...icios.com> [2019-03-26 14:01:08 -0400]:
> ----- On Mar 26, 2019, at 1:45 PM, Szabolcs Nagy nsz@...t70.net wrote:
> > i agree that the current behaviour is not ideal, but
> > iterating over /sys/devices/system/cpu/cpu* may not
> > be correct either.. based on current linux api docs.
> > 
> > i don't understand why is that number different from the
> > cpu set in /sys/devices/system/cpu/possible
> 
> I suspect both iteration over /sys/devices/system/cpu/cpu* and
> content of /sys/devices/system/cpu/possible should provide the
> same result. However, looking at Linux
> Documentation/ABI/testing/sysfs-devices-system-cpu ,
> it appears that /sys/devices/system/cpu/possible was introduced
> in December 2008, whereas /sys/devices/system/cpu/cpu#/ was there
> pre-git history.
> 
> This could explain why glibc uses the iteration method.
> 
> Thoughts ?

as far as i can tell the cpu iteration method is valid,
and that directory list cannot change after boot (is this
guaranteed by the linux abi in the future?), so as long
as /sys is mounted we can get the number, but it's fairly
ugly.. does lttng have fallback code if sysconf returns -1?
if it does maybe musl should just do that (or somebody has
to write cancellation safe directory traversal code)

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