softirq: use ffs() in __do_softirq()

Possible speed improvement of __do_softirq() by using ffs() instead of
using a while loop with an & 1 test then single bit shift.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 22 insertions, 21 deletions

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 8a1e6e104892..ba79bbfe380e 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -229,6 +229,7 @@ asmlinkage void __do_softirq(void)
         struct softirq_action *h;
         bool in_hardirq;
         __u32 pending;
+        int softirq_bit;
         int cpu;
 
         /*
@@ -253,30 +254,30 @@ restart:
 
         h = softirq_vec;
 
-        do {
-                if (pending & 1) {
-                        unsigned int vec_nr = h - softirq_vec;
-                        int prev_count = preempt_count();
-
-                        kstat_incr_softirqs_this_cpu(vec_nr);
-
-                        trace_softirq_entry(vec_nr);
-                        h->action(h);
-                        trace_softirq_exit(vec_nr);
-                        if (unlikely(prev_count != preempt_count())) {
-                                printk(KERN_ERR "huh, entered softirq %u %s %p"
-                                       "with preempt_count %08x,"
-                                       " exited with %08x?\n", vec_nr,
-                                       softirq_to_name[vec_nr], h->action,
-                                       prev_count, preempt_count());
-                                preempt_count_set(prev_count);
-                        }
+        while ((softirq_bit = ffs(pending))) {
+                unsigned int vec_nr;
+                int prev_count;
+
+                h += softirq_bit - 1;
+
+                vec_nr = h - softirq_vec;
+                prev_count = preempt_count();
 
-                        rcu_bh_qs(cpu);
+                kstat_incr_softirqs_this_cpu(vec_nr);
+
+                trace_softirq_entry(vec_nr);
+                h->action(h);
+                trace_softirq_exit(vec_nr);
+                if (unlikely(prev_count != preempt_count())) {
+                        printk(KERN_ERR "huh, entered softirq %u %s %p with preempt_count %08x, exited with %08x?\n",
+                               vec_nr, softirq_to_name[vec_nr], h->action,
+                               prev_count, preempt_count());
+                        preempt_count_set(prev_count);
                 }
+                rcu_bh_qs(cpu);
                 h++;
-                pending >>= 1;
-        } while (pending);
+                pending >>= softirq_bit;
+        }
 
         local_irq_disable();