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    Subject[PATCH 4.4 40/99] sched/core: Fix CPU controller for !RT_GROUP_SCHED
    Date
    From: Juri Lelli <juri.lelli@redhat.com>

    [ Upstream commit a07db5c0865799ebed1f88be0df50c581fb65029 ]

    On !CONFIG_RT_GROUP_SCHED configurations it is currently not possible to
    move RT tasks between cgroups to which CPU controller has been attached;
    but it is oddly possible to first move tasks around and then make them
    RT (setschedule to FIFO/RR).

    E.g.:

    # mkdir /sys/fs/cgroup/cpu,cpuacct/group1
    # chrt -fp 10 $$
    # echo $$ > /sys/fs/cgroup/cpu,cpuacct/group1/tasks
    bash: echo: write error: Invalid argument
    # chrt -op 0 $$
    # echo $$ > /sys/fs/cgroup/cpu,cpuacct/group1/tasks
    # chrt -fp 10 $$
    # cat /sys/fs/cgroup/cpu,cpuacct/group1/tasks
    2345
    2598
    # chrt -p 2345
    pid 2345's current scheduling policy: SCHED_FIFO
    pid 2345's current scheduling priority: 10

    Also, as Michal noted, it is currently not possible to enable CPU
    controller on unified hierarchy with !CONFIG_RT_GROUP_SCHED (if there
    are any kernel RT threads in root cgroup, they can't be migrated to the
    newly created CPU controller's root in cgroup_update_dfl_csses()).

    Existing code comes with a comment saying the "we don't support RT-tasks
    being in separate groups". Such comment is however stale and belongs to
    pre-RT_GROUP_SCHED times. Also, it doesn't make much sense for
    !RT_GROUP_ SCHED configurations, since checks related to RT bandwidth
    are not performed at all in these cases.

    Make moving RT tasks between CPU controller groups viable by removing
    special case check for RT (and DEADLINE) tasks.

    Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Michal Koutný <mkoutny@suse.com>
    Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>
    Acked-by: Tejun Heo <tj@kernel.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: lizefan@huawei.com
    Cc: longman@redhat.com
    Cc: luca.abeni@santannapisa.it
    Cc: rostedt@goodmis.org
    Link: https://lkml.kernel.org/r/20190719063455.27328-1-juri.lelli@redhat.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/core.c | 4 ----
    1 file changed, 4 deletions(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 0e70bfeded7fd..d81bcc6362fff 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -8339,10 +8339,6 @@ static int cpu_cgroup_can_attach(struct cgroup_taskset *tset)
    #ifdef CONFIG_RT_GROUP_SCHED
    if (!sched_rt_can_attach(css_tg(css), task))
    return -EINVAL;
    -#else
    - /* We don't support RT-tasks being in separate groups */
    - if (task->sched_class != &fair_sched_class)
    - return -EINVAL;
    #endif
    }
    return 0;
    --
    2.20.1


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