Possible case of sched_ule never honoring cpu_set affinity?

Justin Teller justin.teller at gmail.com
Tue Feb 24 15:14:57 PST 2009

I believe I've found a case where sched_ule can never honor cpu_set
affinity, and I've attached a suggestion to fix it -- if I'm way off
on how this should be implemented, please let me know!

Imagine the case where sched_affinity is called and the thread is on a
RUNQ on the wrong CPU -- sched_affinity simply exits, relying on
something else in the scheduler to migrate the thread if need be.  The
next time the thread is chosen to run, it runs on the wrong CPU.
Worse, if the thread never goes to sleep or is chosen to be moved by
the load balancer, the thread will continue running on the wrong CPU

Attached is a suggestion of how to change the scheduler to honor
affinity in this case.  With the attached diff, sched_ule will allow a
thread to run on the wrong CPU for one slice.  Then when the thread
moves through sched_switch, if it was running on the wrong CPU, it
will be migrated to the right CPU.  I've written a test where one
thread will bind another thread to a particular CPU (using cpusets)
then waits until it's running on the right CPU before binding it to a
different CPU (and continuing ad nauseam).  Without the change, the
test will sometimes hang waiting for the second thread to get on the
correct CPU -- with the change, it works every time.

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