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Date: Tue, 2 Dec 2014 17:33:04 -0800
From: Isaac Dunham <ibid.ag@...il.com>
To: musl@...ts.openwall.com
Subject: Re: __sched_cpucount returns garbage
On Tue, Dec 02, 2014 at 07:11:15PM -0500, Rich Felker wrote:
> On Sun, Nov 30, 2014 at 12:38:46PM +0100, Szabolcs Nagy wrote:
> > * Isaac Dunham <ibid.ag@...il.com> [2014-11-29 15:36:33 -0800]:
> > > I noticed that nproc ended up on the toybox TODO list (via Tizen), and went
> > > poking about via strace and ltrace to see where it got the cpu count from.
> > >
> > > In the process, I discovered that __sched_cpucount is returning garbage;
> >
> > works here as expected:
> >
> > #define _GNU_SOURCE
> > #include <sched.h>
> > int main()
> > {
> > cpu_set_t s = {0};
> > CPU_SET(3, &s);
> > CPU_SET(7, &s);
> > CPU_SET(24, &s);
> > return __sched_cpucount(sizeof s, &s);
> > }
> >
> > returns 3
> >
> > > on Alpine Linux on my N270-based netbook (1 physical core but
> > > hyperthreading makes it look like 2),
> > > nproc
> > > outputs a random number of CPUs ranging from 413 to 472.
> >
> > see where the cpu_set_t argument comes from
> > (most likely sched_getaffinity syscall)
> > then see why that is broken
> >
> > __sched_cpucount just counts bit flags
>
> Is it possible that the macros from sched.h are using it wrong, or
> that nproc is using __sched_cpucount directly rather than using the
> sched.h macros and expecting different behavior from it (perhaps a
> mismatch between the musl and glibc behavior, like counting bits vs
> bytes vs longs)?
>
> Rich
I have no idea what it's doing; after reading the source, I have *less*
of an understanding, since it's got half a dozen #ifdefs in the relevant
code (in lib/nproc.c).
But I can say that it's returning the result of __sched_cpucount without
modification (the return matches the output of nproc).
OK, rereading it:
We're probably using HAVE_SCHED_GETAFFINITY_LIKE_GLIBC, and CPU_COUNT is
defined.
So it ostensibly should be more-or-less:
if (sched_getaffinity (0, sizeof (set), &set) == 0)
{
unsigned long count;
count = CPU_COUNT(&set);
if (count > 0)
return count;
}
BUT... isolating that snippet gives me the expected results...if
I initialize set to 0, which they *don't*.
So I guess it's the missing initialization.
Thanks,
Isaac Dunham
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