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    SubjectRe: 2.6.16 - sys_sched_getaffinity & hotplug
    Jack Steiner wrote:
    >
    > It appears if CONFIG_HOTPLUG_CPU is enabled, then all possible
    > cpus (0 .. NR_CPUS-1) are set in the cpu_possible_map on IA64.

    That's too bad...


    > sched_getaffinity() returns the cpu_possible_map and'd with the current
    > task p->cpus_allowed. The default cpus_allowed is all ones.
    >
    > This is causing problems for apps that use sched_get_sched_affinity()
    > to determine which cpus that they are allowed to run on.

    How? Are these apps expecting all set bits to correspond to online
    cpus?


    > The call to sched_getaffinity returns:
    >
    > (from strace on a 2 cpu system with NR_CPUS = 512)
    > sched_getaffinity(0, 1024, { ffffffffffffffff, ffffff ...
    >
    >
    >
    > The man page for sched_getaffinity() is ambiguous. It says:
    > - A set bit corresponds to a legally schedulable CPU
    >
    > But it also says:
    > - Usually, all bits in the mask are set.
    >
    >
    > Should the following change be made to sched_getaffinity().
    >
    > Index: linux/kernel/sched.c
    > ===================================================================
    > --- linux.orig/kernel/sched.c 2006-01-25 08:50:21.401747695 -0600
    > +++ linux/kernel/sched.c 2006-01-27 16:57:24.504871895 -0600
    > @@ -4031,7 +4031,7 @@ long sched_getaffinity(pid_t pid, cpumas
    > goto out_unlock;
    >
    > retval = 0;
    > - cpus_and(*mask, p->cpus_allowed, cpu_possible_map);
    > + cpus_and(*mask, p->cpus_allowed, cpu_online_map);


    I don't think so.

    For one, that would be mucking around with a kernel/userspace ABI, I
    guess.

    Additionally, it would mean that the result of sched_getaffinity would
    vary with the number of online cpus in the system, which I don't think
    is desirable.
    -
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