sparc64: fix and optimize irq distribution

irq_choose_cpu() should compare the affinity mask against cpu_online_map
rather than CPU_MASK_ALL, since irq_select_affinity() sets the interrupt's
affinity mask to cpu_online_map "and" CPU_MASK_ALL (which ends up being
just cpu_online_map).  The mask comparison in irq_choose_cpu() will always
fail since the two masks are not the same.  So the CPU chosen is the first CPU
in the intersection of cpu_online_map and CPU_MASK_ALL, which is always CPU0.
That means all interrupts are reassigned to CPU0...

Distributing interrupts to CPUs in a linearly increasing round robin fashion
is not optimal for the UltraSPARC T1/T2.  Also, the irq_rover in
irq_choose_cpu() causes an interrupt to be assigned to a different
processor each time the interrupt is allocated and released.  This may lead
to an unbalanced distribution over time.

A static mapping of interrupts to processors is done to optimize and balance
interrupt distribution.  For the T1/T2, interrupts are spread to different
cores first, and then to strands within a core.

The following is some benchmarks showing the effects of interrupt
distribution on a T2.  The test was done with iperf using a pair of T5220
boxes, each with a 10GBe NIU (XAUI) connected back to back.

  TCP     | Stock       Linear RR IRQ  Optimized IRQ
  Streams | 2.6.30-rc5  Distribution   Distribution
          | GBits/sec   GBits/sec      GBits/sec
  --------+-----------------------------------------
    1       0.839       0.862          0.868
    8       1.16        4.96           5.88
   16       1.15        6.40           8.04
  100       1.09        7.28           8.68

Signed-off-by: Hong H. Pham <hong.pham@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 25 deletions

diff --git a/arch/sparc/kernel/irq_64.c b/arch/sparc/kernel/irq_64.c
index e5e78f9cfc95..bd075054942b 100644
--- a/arch/sparc/kernel/irq_64.c
+++ b/arch/sparc/kernel/irq_64.c
@@ -45,6 +45,7 @@
 #include <asm/cacheflush.h>
 
 #include "entry.h"
+#include "cpumap.h"
 
 #define NUM_IVECS       (IMAP_INR + 1)
 
@@ -256,35 +257,13 @@ static int irq_choose_cpu(unsigned int virt_irq)
         int cpuid;
 
         cpumask_copy(&mask, irq_desc[virt_irq].affinity);
-        if (cpus_equal(mask, CPU_MASK_ALL)) {
-                static int irq_rover;
-                static DEFINE_SPINLOCK(irq_rover_lock);
-                unsigned long flags;
-
-                /* Round-robin distribution... */
-        do_round_robin:
-                spin_lock_irqsave(&irq_rover_lock, flags);
-
-                while (!cpu_online(irq_rover)) {
-                        if (++irq_rover >= nr_cpu_ids)
-                                irq_rover = 0;
-                }
-                cpuid = irq_rover;
-                do {
-                        if (++irq_rover >= nr_cpu_ids)
-                                irq_rover = 0;
-                } while (!cpu_online(irq_rover));
-
-                spin_unlock_irqrestore(&irq_rover_lock, flags);
+        if (cpus_equal(mask, cpu_online_map)) {
+                cpuid = map_to_cpu(virt_irq);
         } else {
                 cpumask_t tmp;
 
                 cpus_and(tmp, cpu_online_map, mask);
-
-                if (cpus_empty(tmp))
-                        goto do_round_robin;
-
-                cpuid = first_cpu(tmp);
+                cpuid = cpus_empty(tmp) ? map_to_cpu(virt_irq) : first_cpu(tmp);
         }
 
         return cpuid;