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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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