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If load_balance() fails to migrate any tasks because all tasks were affined, load_balance() removes the source CPU from consideration and attempts to redo and balance among the new subset of CPUs. There is a bug in this code path where the algorithm considers all active CPUs in the system (minus the source that was just masked out). This is not valid for two reasons: some active CPUs may not be in the current scheduling domain and one of the active CPUs is dst_cpu. These CPUs should not be considered, as we cannot pull load from them. Instead of failing out of load_balance(), we may end up redoing the search with no valid CPUs and incorrectly concluding the domain is balanced. Additionally, if the group_imbalance flag was just set, it may also be incorrectly unset, thus the flag will not be seen by other CPUs in future load_balance() runs as that algorithm intends. Fix the check by removing CPUs not in the current domain and the dst_cpu from considertation, thus limiting the evaluation to valid remaining CPUs from which load might be migrated. Co-authored-by: Austin Christ <austinwc@codeaurora.org> Co-authored-by: Dietmar Eggemann <dietmar.eggemann@arm.com> Tested-by: Tyler Baicar <tbaicar@codeaurora.org> Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Austin Christ <austinwc@codeaurora.org> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Timur Tabi <timur@codeaurora.org> Link: http://lkml.kernel.org/r/1496863138-11322-2-git-send-email-jhugo@codeaurora.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.