Re: libcruft/sysconf_cpus.c patch

Indan Zupancic wrote:

> So I'd argue that dietlibc is free to do what it wants here. Everyone using
> this interface wants to know how many cpus they can actually use anyway.

If they want to know how many they're currently setup for, they can use
getaffinity.  The point of having a getconf call is to get below that to
what the OS has available.

Then if the process has enough privilege (which is no special priv on
Linux, by default), it can use setaffinity to turn back on processors it
was started without.  Even if it has insufficient privilege, it can
social engineer: "dear user, this will perform better if you run it with
all CPUs enabled".

> This you can find out with with one sched_getaffinity() call. Then there's
> the theoretical case of more cpus becoming available after the call, but as
> this is very unlikely and so obscure that I don't think dietlibc should
> bother getting this corner case right.

This is the wrong year to be calling it obscure: virtualization
platforms are busily adding hot-plug VCPU support.

> > There are at least 3 numbers one *could* want to know: (1) # CPUs in the
> > hardware, (2) # CPUs activated by the system (e.g. if the OS has CPU
> > licenses, or detected some are broken), and (3) # CPUs *this* process is
> > allowed to access.  The doc says (to me) #2.
> 
> Knowing 1 gives you nothing useful, neither does 2 if 3 is different.

#2 is like "ulimit -H #cpus" and #3 is "ulimit -S #cpus", it is worth
while knowing the difference.

> > #if !defined(SLASH_PROC_OK) || defined(__arm__)
> > int __sc_nr_cpus() {
> >   return 1;	/* kludge kludge ;-) */
> > }
> 
> Some new ARMs have SMP support.

Ok.  That was just carried in from old code.

> Shouldn't you check if nr is greater than 0, to catch untested platforms?

Probably.  The old code didn't.

> For comparison, the same using sched_getaffinity() would look something
> like this:
> 
> #include <sched.h>
> 
> #define NR 8
> 
> int __sc_nr_cpus(void)
> {
> 	int i, c;
> 	unsigned long v;
> 	unsigned long m[NR];
> 
> 	if (sched_getaffinity(0, sizeof(m), &m))
> 		return 1;
> 	for (c = 0, i = 0; i < NR; i++){
> 		/* Peter Wegner/Derrick Lehmer/Brian Kernighan's method */
> 		for (v = m[i]; v; c++){
> 		  v &= v - 1; // clear the least significant bit set
> 		}
> 	}
> 	return c;
> }

Pleasingly simple, at least.

Looking upwards at sysconf.c (caller of __sc_nr_cpus()), I see there's
little attempt to read out the real system at hand.  So I see I'm
inappropriately arguing for accuracy in a setting of general laxness.
Never mind.

>Bela<