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>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210402132037.574661-1-npiggin@gmail.com