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    Subject[PATCH v2 5/7] Docs, cpusets: Update the cpuset documentation
    Date
    Add documentation for the newly introduced cpuset.actual_cpus file and
    describe the new semantics for updating cpusets upon CPU hotplug.

    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    Cc: stable@vger.kernel.org
    ---

    Documentation/cgroups/cpusets.txt | 43 +++++++++++++++++++++++++------------
    1 files changed, 29 insertions(+), 14 deletions(-)

    diff --git a/Documentation/cgroups/cpusets.txt b/Documentation/cgroups/cpusets.txt
    index cefd3d8..374b9d2 100644
    --- a/Documentation/cgroups/cpusets.txt
    +++ b/Documentation/cgroups/cpusets.txt
    @@ -168,7 +168,12 @@ Each cpuset is represented by a directory in the cgroup file system
    containing (on top of the standard cgroup files) the following
    files describing that cpuset:

    - - cpuset.cpus: list of CPUs in that cpuset
    + - cpuset.cpus: list of CPUs in that cpuset, as set by the user;
    + the kernel will not alter this upon CPU hotplug;
    + this file has read/write permissions
    + - cpuset.actual_cpus: list of CPUs actually available for the tasks in the
    + cpuset; the kernel can change this in the event of
    + CPU hotplug; this file is read-only
    - cpuset.mems: list of Memory Nodes in that cpuset
    - cpuset.memory_migrate flag: if set, move pages to cpusets nodes
    - cpuset.cpu_exclusive flag: is cpu placement exclusive?
    @@ -640,16 +645,25 @@ prior 'cpuset.mems' setting, will not be moved.

    There is an exception to the above. If hotplug functionality is used
    to remove all the CPUs that are currently assigned to a cpuset,
    -then all the tasks in that cpuset will be moved to the nearest ancestor
    -with non-empty cpus. But the moving of some (or all) tasks might fail if
    -cpuset is bound with another cgroup subsystem which has some restrictions
    -on task attaching. In this failing case, those tasks will stay
    -in the original cpuset, and the kernel will automatically update
    -their cpus_allowed to allow all online CPUs. When memory hotplug
    -functionality for removing Memory Nodes is available, a similar exception
    -is expected to apply there as well. In general, the kernel prefers to
    -violate cpuset placement, over starving a task that has had all
    -its allowed CPUs or Memory Nodes taken offline.
    +then the cpuset hierarchy is traversed, searching for the nearest
    +ancestor whose cpu mask has atleast one online cpu. Then the tasks in
    +the empty cpuset will be run on the cpus specified in that ancestor's cpu mask.
    +Note that during CPU hotplug operations, the tasks in a cpuset will not
    +be moved from one cpuset to another; only the the cpu mask of that cpuset
    +will be updated to ensure that there is atleast one online cpu, by trying
    +to closely resemble the cpu mask of the nearest non-empty ancestor containing
    +online cpus.
    +
    +When memory hotplug functionality for removing Memory Nodes is available,
    +if all the memory nodes currently assigned to a cpuset are removed via
    +hotplug, then all the tasks in that cpuset will be moved to the nearest
    +ancestor with non-empty memory nodes. But the moving of some (or all)
    +tasks might fail if cpuset is bound with another cgroup subsystem which
    +has some restrictions on task attaching. In this failing case, those
    +tasks will stay in the original cpuset, and the kernel will automatically
    +update their mems_allowed to allow all online nodes.
    +In general, the kernel prefers to violate cpuset placement, over starving
    +a task that has had all its allowed CPUs or Memory Nodes taken offline.

    There is a second exception to the above. GFP_ATOMIC requests are
    kernel internal allocations that must be satisfied, immediately.
    @@ -730,9 +744,10 @@ cgroup.event_control cpuset.memory_spread_page
    cgroup.procs cpuset.memory_spread_slab
    cpuset.cpu_exclusive cpuset.mems
    cpuset.cpus cpuset.sched_load_balance
    -cpuset.mem_exclusive cpuset.sched_relax_domain_level
    -cpuset.mem_hardwall notify_on_release
    -cpuset.memory_migrate tasks
    +cpuset.actual_cpus cpuset.sched_relax_domain_level
    +cpuset.mem_exclusive notify_on_release
    +cpuset.mem_hardwall tasks
    +cpuset.memory_migrate

    Reading them will give you information about the state of this cpuset:
    the CPUs and Memory Nodes it can use, the processes that are using


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