Date: Tue, 26 Mar 2019 14:01:08 -0400 (EDT) From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> To: Szabolcs Nagy <nsz@...t70.net> 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 ----- On Mar 26, 2019, at 1:45 PM, Szabolcs Nagy nsz@...t70.net wrote: > * 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 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 ? Thanks, Mathieu > > it seems any upper bound on the number of cpus would be > valid but it's not clear how to provide that guarantee. -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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