Correct, Ingo objected to the fastpath overhead.Works great. Thanks
Could you please try the below patch which tries to address the issue
differently.
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Subject: sched: Fix balance vs hotplug race
From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Date: Wed Nov 25 13:31:39 CET 2009
Since (e761b77: cpu hotplug, sched: Introduce cpu_active_map and redo
sched domain managment) we have cpu_active_mask which is suppose to
rule scheduler migration and load-balancing, except it never did.
The particular problem being solved here is a crash in
try_to_wake_up() where select_task_rq() ends up selecting an offline
cpu because select_task_rq_fair() trusts the sched_domain tree to reflect
the current state of affairs, similarly select_task_rq_rt() trusts the
root_domain.
However, the sched_domains are updated from CPU_DEAD, which is after
the cpu is taken offline and after stop_machine is done. Therefore it
can race perfectly well with code assuming the domains are right.
Cure this by building the domains from cpu_active_mask on
CPU_DOWN_PREPARE.