Date: Tue, 2 Apr 2019 18:38:18 -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 Apr 2, 2019, at 3:02 PM, Szabolcs Nagy nsz@...t70.net wrote: > * 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) Nope, the lttng-ust ring buffer won't be usable anyway. I *think* we need to improve the error handling for that scenario within our consumer daemon which is responsible for buffer allocation. I don't think we properly error out if the internal variable tracking the number of CPUs in the system stays at its unitialized value of 0. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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