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Rik van Riel authored
There is a subtle interaction between the logic introduced in commit e63da036 ("sched/numa: Allow task switch if load imbalance improves"), the way the load balancer counts the load on each NUMA node, and the way NUMA hinting faults are done. Specifically, the load balancer only counts currently running tasks in the load, while NUMA hinting faults may cause tasks to stop, if the page is locked by another task. This could cause all of the threads of a large single instance workload, like SPECjbb2005, to migrate to the same NUMA node. This was possible because occasionally they all fault on the same few pages, and only one of the threads remains runnable. That thread can move to the process's preferred NUMA node without making the imbalance worse, because nothing else is running at that time. The fix is to check the direction of the net moving of load, and to refuse a NUMA move if it would cause the system to move past the point of balance. In an unbalanced state, only moves that bring us closer to the balance point are allowed. Signed-off-by:
Rik van Riel <riel@redhat.com> Signed-off-by:
Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: mgorman@suse.de Link: http://lkml.kernel.org/r/20150203165648.0e9ac692@annuminas.surriel.comSigned-off-by:
Ingo Molnar <mingo@kernel.org>
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