[PATCH] genirq: speedup show_interrupts()

[Eric Dumazet]

Since commit 425a5072dcd1 ("genirq: Free irq_desc with rcu"),
show_interrupts() can be switched to rcu locking, which removes
possible contention on sparse_irq_lock.

The per_cpu count scan and print can be done without holding
desc spinlock.

And there is no need to call kstat_irqs_cpu() and abuse
irq_to_desc() while holding rcu read lock, since desc and
desc->kstat_irqs wont disappear or change.

Signed-off-by: Eric Dumazet <eduma...@google.com>
---
 kernel/irq/proc.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/kernel/irq/proc.c b/kernel/irq/proc.c
index 
37eda10f5c362fb173f4280fe77fd24ebc8c195f..da9addb8d655719cbd67a59e5d8535670e28f885
 100644
--- a/kernel/irq/proc.c
+++ b/kernel/irq/proc.c
@@ -475,22 +475,24 @@ int show_interrupts(struct seq_file *p, void *v)
                seq_putc(p, '\n');
        }
 
-       irq_lock_sparse();
+       rcu_read_lock();
        desc = irq_to_desc(i);
        if (!desc)
                goto outsparse;
 
-       raw_spin_lock_irqsave(&desc->lock, flags);
-       for_each_online_cpu(j)
-               any_count |= kstat_irqs_cpu(i, j);
-       action = desc->action;
-       if ((!action || irq_desc_is_chained(desc)) && !any_count)
-               goto out;
+       if (desc->kstat_irqs)
+               for_each_online_cpu(j)
+                       any_count |= *per_cpu_ptr(desc->kstat_irqs, j);
+
+       if ((!desc->action || irq_desc_is_chained(desc)) && !any_count)
+               goto outsparse;
 
        seq_printf(p, "%*d: ", prec, i);
        for_each_online_cpu(j)
-               seq_printf(p, "%10u ", kstat_irqs_cpu(i, j));
+               seq_printf(p, "%10u ", desc->kstat_irqs ?
+                                       *per_cpu_ptr(desc->kstat_irqs, j) : 0);
 
+       raw_spin_lock_irqsave(&desc->lock, flags);
        if (desc->irq_data.chip) {
                if (desc->irq_data.chip->irq_print_chip)
                        desc->irq_data.chip->irq_print_chip(&desc->irq_data, p);
@@ -511,6 +513,7 @@ int show_interrupts(struct seq_file *p, void *v)
        if (desc->name)
                seq_printf(p, "-%-8s", desc->name);
 
+       action = desc->action;
        if (action) {
                seq_printf(p, "  %s", action->name);
                while ((action = action->next) != NULL)
@@ -518,10 +521,9 @@ int show_interrupts(struct seq_file *p, void *v)
        }
 
        seq_putc(p, '\n');
-out:
        raw_spin_unlock_irqrestore(&desc->lock, flags);
 outsparse:
-       irq_unlock_sparse();
+       rcu_read_unlock();
        return 0;
 }
 #endif
-- 
2.18.0.rc1.244.gcf134e6275-goog