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SubjectWhy is cpuset_cpus_allowed_fallback() necessary?
Hello, guys.

I'm wondering why cpuset_cpus_allowed_fallback() is necessary. This
is called from, e.g., try_to_wake_up()->select_task_rq() when none of
the cpus in ->cpus_allowed is useable. The cpuset callback invokes
do_set_cpus_allowed() w/ the cpuset's cpus_allowed. This was added by
the following commit,

commit 9084bb8246ea935b98320554229e2f371f7f52fa
Author: Oleg Nesterov <>
Date: Mon Mar 15 10:10:27 2010 +0100

sched: Make select_fallback_rq() cpuset friendly

Introduce cpuset_cpus_allowed_fallback() helper to fix the cpuset problems
with select_fallback_rq(). It can be called from any context and can't use
any cpuset locks including task_lock(). It is called when the task doesn't
have online cpus in ->cpus_allowed but ttwu/etc must be able to find a
suitable cpu.

The problem is, nothing's explaining what "the cpuset problems with
select_fallback_rq()" are. Oleg, do you remember? Why do we need



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