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    Subject[PATCH] genirq: reduce irqdebug bouncing cachelines
    Date
    note_interrupt increments desc->irq_count for each interrupt even for
    percpu interrupt handlers, even when they are handled successfully. This
    causes cacheline bouncing and limits scalability.

    Instead of incrementing irq_count every time, only start incrementing it
    after seeing an unhandled irq, which should avoid the cache line
    bouncing in the common path.

    This actually should give better consistency in handling misbehaving
    irqs too, because instead of the first unhandled irq arriving at an
    arbitrary point in the irq_count cycle, its arrival will begin the
    irq_count cycle.

    Cédric reports the result of his IPI throughput test:

    Millions of IPIs/s
    ----------- --------------------------------------
    upstream upstream patched
    chips cpus default noirqdebug default (irqdebug)
    ----------- -----------------------------------------
    1 0-15 4.061 4.153 4.084
    0-31 7.937 8.186 8.158
    0-47 11.018 11.392 11.233
    0-63 11.460 13.907 14.022
    2 0-79 8.376 18.105 18.084
    0-95 7.338 22.101 22.266
    0-111 6.716 25.306 25.473
    0-127 6.223 27.814 28.029

    Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
    ---
    kernel/irq/spurious.c | 4 ++++
    1 file changed, 4 insertions(+)

    diff --git a/kernel/irq/spurious.c b/kernel/irq/spurious.c
    index f865e5f4d382..c481d8458325 100644
    --- a/kernel/irq/spurious.c
    +++ b/kernel/irq/spurious.c
    @@ -403,6 +403,10 @@ void note_interrupt(struct irq_desc *desc, irqreturn_t action_ret)
    desc->irqs_unhandled -= ok;
    }

    + if (likely(!desc->irqs_unhandled))
    + return;
    +
    + /* Now getting into unhandled irq detection */
    desc->irq_count++;
    if (likely(desc->irq_count < 100000))
    return;
    --
    2.23.0
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