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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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