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    SubjectRe: [RFC][PATCH 4/7] smp: Optimize send_call_function_single_ipi()
    On Wed, May 27, 2020 at 11:56:45AM +0200, Peter Zijlstra wrote:

    > This is rcu_is_cpu_rrupt_from_idle()'s lockdep_assert_in_irq() tripping
    > up (it's comment is obviously a bit antiquated).
    >
    > Now, if I read that code correctly, it actually relies on
    > rcu_irq_enter() and thus really wants to be in an interrupt. Is there
    > any way this code can be made to work from the new context too?
    >
    > After all, all that really is different is not having gone throught he
    > bother of setting up the IRQ context, nothing else changed -- it just so
    > happens you actually relied on that ;/

    At first glance, something like the below could work. But obviously I
    might have overlooked something more subtle than a brick :-)

    ---

    diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    index 90c8be22d57a..0792c032a972 100644
    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -426,8 +426,11 @@ EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);
    */
    static int rcu_is_cpu_rrupt_from_idle(void)
    {
    - /* Called only from within the scheduling-clock interrupt */
    - lockdep_assert_in_irq();
    + /*
    + * Usually called from the tick; but also used from smp_call_function()
    + * for expedited grace periods.
    + */
    + lockdep_assert_irqs_disabled();

    /* Check for counter underflows */
    RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) < 0,
    @@ -435,8 +438,11 @@ static int rcu_is_cpu_rrupt_from_idle(void)
    RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0,
    "RCU dynticks_nmi_nesting counter underflow/zero!");

    - /* Are we at first interrupt nesting level? */
    - if (__this_cpu_read(rcu_data.dynticks_nmi_nesting) != 1)
    + /*
    + * Are we at first interrupt nesting level? -- or below, when running
    + * directly from the idle loop itself.
    + */
    + if (__this_cpu_read(rcu_data.dynticks_nmi_nesting) > 1)
    return false;

    /* Does CPU appear to be idle from an RCU standpoint? */
    \
     
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      Last update: 2020-05-27 12:20    [W:4.188 / U:0.276 seconds]
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