Date: Tue, 26 Mar 2019 18:45:41 +0100 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Cc: Michael Jeanson <mjeanson@...icios.com>, Richard Purdie <richard.purdie@...uxfoundation.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Jonathan Rajotte-Julien <jonathan.rajotte-julien@...icios.com> Subject: Re: sysconf(_SC_NPROCESSORS_CONF) returns the wrong value * Jonathan Rajotte-Julien <jonathan.rajotte-julien@...icios.com> [2019-03-26 12:23:34 -0400]: > > i think we need to know why does a process care if musl returns > > the wrong number? or what are the valid uses of such a number? > > (there are heterogeous systems like arm big-little, numa systems > > with many sockets, containers, virtualization,.. how deep may a > > user process need to go down in this rabbit hole?) > > Does the answers from Mathieu Desnoyers  and Florian Weimer  fit the bill? yes > >  https://www.openwall.com/lists/musl/2019/03/16/3 >  https://www.openwall.com/lists/musl/2019/03/19/1 > > > > note that most of /sys/devices/system/cpu/* is documented under > > Documentation/ABI/testing in linux, not in Documentation/ABI/stable > > and the format is not detailed, and some apis (e.g. /proc/cpuinfo) > > are known to be different on android (and grsec?) kernels it may > > be unmounted during early boot or in chroots, so sysfs parsing is > > only done when really necessary. > > For what it's worth, uclibc and uclibc-ng seem to iterate over > /sys/devices/system/cpu/* and fallback on online calculation if necessary. > > https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/tree/libc/unistd/sysconf.c#n102 > > In the mean time, we implemented a fallback similar to this when we do not "know" > the libc used (since musl does not come with __musl__, I read the reasons why, > no need to discuss this). > > Not sure of the direction musl should take but I strongly believe that the > behaviour regarding _SC_NPROCESSORS_CONF is not the appropriate one. 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 it seems any upper bound on the number of cpus would be valid but it's not clear how to provide that guarantee.
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