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    Subject[PATCH 4.19 104/110] rcu: Make need_resched() respond to urgent RCU-QS needs
    Date
    4.19-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    commit 92aa39e9dc77481b90cbef25e547d66cab901496 upstream.

    The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent
    need for an RCU quiescent state from the force-quiescent-state processing
    within the grace-period kthread to context switches and to cond_resched().
    Unfortunately, such urgent needs are not communicated to need_resched(),
    which is sometimes used to decide when to invoke cond_resched(), for
    but one example, within the KVM vcpu_run() function. As of v4.15, this
    can result in synchronize_sched() being delayed by up to ten seconds,
    which can be problematic, to say nothing of annoying.

    This commit therefore checks rcu_dynticks.rcu_urgent_qs from within
    rcu_check_callbacks(), which is invoked from the scheduling-clock
    interrupt handler. If the current task is not an idle task and is
    not executing in usermode, a context switch is forced, and either way,
    the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current
    task is an idle task, then RCU's dyntick-idle code will detect the
    quiescent state, so no further action is required. Similarly, if the
    task is executing in usermode, other code in rcu_check_callbacks() and
    its called functions will report the corresponding quiescent state.

    Reported-by: Marius Hillenbrand <mhillenb@amazon.de>
    Reported-by: David Woodhouse <dwmw2@infradead.org>
    Suggested-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    [ paulmck: Backported to make patch apply cleanly on older versions. ]
    Tested-by: Marius Hillenbrand <mhillenb@amazon.de>
    Cc: <stable@vger.kernel.org> # 4.12.x - 4.19.x
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/rcu/tree.c | 9 +++++++++
    1 file changed, 9 insertions(+)

    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -2662,6 +2662,15 @@ void rcu_check_callbacks(int user)
    rcu_bh_qs();
    }
    rcu_preempt_check_callbacks();
    + /* The load-acquire pairs with the store-release setting to true. */
    + if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) {
    + /* Idle and userspace execution already are quiescent states. */
    + if (!rcu_is_cpu_rrupt_from_idle() && !user) {
    + set_tsk_need_resched(current);
    + set_preempt_need_resched();
    + }
    + __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
    + }
    if (rcu_pending())
    invoke_rcu_core();


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