[PATCH v6 0/4] CPU hotplug, cpusets, suspend/resume: Fixes,cleanups and optimizations

[Srivatsa S. Bhat]

Currently the kernel doesn't handle cpusets properly during suspend/resume.
After a resume, all non-root cpusets end up having only 1 cpu (the boot cpu),
causing massive performance degradation of workloads. One major user of cpusets
is libvirt, which means that after a suspend/hibernation cycle, all VMs
suddenly end up running terribly slow!

Also, the kernel moves the tasks from one cpuset to another during CPU hotplug
in the suspend/resume path, leading to a task-management nightmare after
resume.

Patch 1 fixes this by keeping cpusets unmodified in the suspend/resume path.
But to ensure we don't trip over, it keeps the sched domains updated during
every CPU hotplug in the s/r path.
This is a long standing issue and we need to fix up stable kernels too.

The rest of the patches in the series are mostly cleanups/optimizations.

Changelog:

v6: Drop patch 4 (cpusets: Update tasks' cpus_allowed mask upon updates to
    root cpuset) considering how tasks in the root cpuset must be dealt with.
    No changes to other patches.

v5 : Don't do explicit save/restore of cpusets' cpu masks during s/r, to avoid
     the overhead of a new per-cpuset cpu mask to help with that. Also, ensure
     that the sched domains are updated on each hotplug.
     The s/r fix (patch 1) is now distinct from the cleanups (patches 2-5).

v4 : (Never had this version. I skipped from v3 to v5 accidentally).

v3 : Explicitly save and restore cpusets' cpu masks during s/r. Don't alter
     the semantics of regular cpu hotplug.
     http://thread.gmane.org/gmane.linux.kernel/1296339

v2 : http://thread.gmane.org/gmane.linux.documentation/4805/

v1 : thread.gmane.org/gmane.linux.kernel/1250097/

--
 Srivatsa S. Bhat (4):
      CPU hotplug, cpusets, suspend: Don't modify cpusets during suspend/resume
      cpusets, hotplug: Implement cpuset tree traversal in a helper function
      cpusets, hotplug: Restructure functions that are invoked during hotplug
      cpusets: Remove/update outdated comments


  include/linux/cpuset.h |    4 +
 kernel/cpuset.c        |  130 ++++++++++++++++++++++++++++++++++--------------
 kernel/sched/core.c    |   44 ++++++++++++++--
 3 files changed, 132 insertions(+), 46 deletions(-)



Thanks,
Srivatsa S. Bhat
IBM Linux Technology Center

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