Re: For review: pthread_attr_setaffinity_np.3

[Loic Domaigne <tech-Z4JMKDdsf89Wk0Htik3J/w@public.gmane.org>]

Hi Bert!


>>> .TP
>>> .B ENOMEM
>>> .RB ( pthread_attr_setaffinity_np ())
>>> Could not allocate memory.

>> Correct. However, the glibc looks somewhat suspicious to me: I had expected
>> the memory allocated by iattr->cpuset to be bounded (by the size of the
>> underlying kernel structure).
> But this kernel structure is not bounded, it depends on the compile
> time constant CONFIG_NR_CPUS (or in the future on the kernel runtime
> variable nr_cpus_id), therefore glibc can't know this a-priori. (but I
> haven't looked into the code)

AFAICS, the kernel structure size is determined at runtime by glibc on 
the first call of pthread_setaffinity_np() or 
pthread_attr_setaffinity_np(). Please refer to the function 
__determine_cpumask_size() in 
nptl/sysdeps/unix/sysv/linux/pthread_setaffinity.c

Once you know this structure size, there is no reason to allocate a 
bigger room for the attribute cpuset iattr->cpuset. As of glibc-2.7, the 
memory size allocated is based on the cupsetsize argument passed to 
pthread_attr_setaffinity_np() without respect to the underlying kernel 
structure.

Of course the Linux & Glibc folks might have some future plans that may 
not be easily inferred from the actual source.

Loïc.
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