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    Subject[tip:sched/urgent] sched/rt: Do not pull from current CPU if only one cpu to pull
    Commit-ID:  f80a6308b1100d431e27afb61aa90d752df20aae
    Gitweb: https://git.kernel.org/tip/f80a6308b1100d431e27afb61aa90d752df20aae
    Author: Steven Rostedt <rostedt@goodmis.org>
    AuthorDate: Sat, 2 Dec 2017 13:04:54 -0500
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Mon, 11 Dec 2017 17:46:58 +0100

    sched/rt: Do not pull from current CPU if only one cpu to pull

    Daniel Wagner reported a crash on the beaglebone black. This is a
    single CPU architecture, and does not have a functional:
    arch_send_call_function_single_ipi() and can crash if that is called.

    As it only has one CPU, it shouldn't be called, but if the kernel is
    compiled for SMP, the push/pull RT scheduling logic now calls it for
    irq_work if the one CPU is overloaded, it can use that function to call
    itself and crash the kernel.

    Ideally, we should disable the SCHED_FEAT(RT_PUSH_IPI) if the system
    only has a single CPU. But SCHED_FEAT is a constant if sched debugging
    is turned off. Another fix can also be used, and this should also help
    with normal SMP machines. That is, do not initiate the pull code if
    there's only one RT overloaded CPU, and that CPU happens to be the
    current CPU that is scheduling in a lower priority task.

    Even on a system with many CPUs, if there's many RT tasks waiting to
    run on a single CPU, and that CPU schedules in another RT task of lower
    priority, it will initiate the PULL logic in case there's a higher
    priority RT task on another CPU that is waiting to run. But if there is
    no other CPU with waiting RT tasks, it will initiate the RT pull logic
    on itself (as it still has RT tasks waiting to run). This is a wasted
    effort.

    Not only does this help with SMP code where the current CPU is the only
    one with RT overloaded tasks, it should also solve the issue that
    Daniel encountered, because it will prevent the PULL logic from
    executing, as there's only one CPU on the system, and the check added
    here will cause it to exit the RT pull code.

    Reported-by: Daniel Wagner <wagi@monom.org>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Acked-by: Peter Zijlstra <peterz@infradead.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: linux-rt-users <linux-rt-users@vger.kernel.org>
    Cc: stable@vger.kernel.org
    Fixes: 4bdced5c9 ("sched/rt: Simplify the IPI based RT balancing logic")
    Link: http://lkml.kernel.org/r/20171202130454.4cbbfe8d@vmware.local.home
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    kernel/sched/rt.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
    index 4056c19..665ace2 100644
    --- a/kernel/sched/rt.c
    +++ b/kernel/sched/rt.c
    @@ -2034,8 +2034,9 @@ static void pull_rt_task(struct rq *this_rq)
    bool resched = false;
    struct task_struct *p;
    struct rq *src_rq;
    + int rt_overload_count = rt_overloaded(this_rq);

    - if (likely(!rt_overloaded(this_rq)))
    + if (likely(!rt_overload_count))
    return;

    /*
    @@ -2044,6 +2045,11 @@ static void pull_rt_task(struct rq *this_rq)
    */
    smp_rmb();

    + /* If we are the only overloaded CPU do nothing */
    + if (rt_overload_count == 1 &&
    + cpumask_test_cpu(this_rq->cpu, this_rq->rd->rto_mask))
    + return;
    +
    #ifdef HAVE_RT_PUSH_IPI
    if (sched_feat(RT_PUSH_IPI)) {
    tell_cpu_to_push(this_rq);
    \
     
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