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Subject[PATCH 1/3] softirq: Use ffs in __do_softirq
Date
Possible speed improvement of the __do_softirq function 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>
---
kernel/softirq.c | 43 ++++++++++++++++++++++---------------------
1 file changed, 22 insertions(+), 21 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 11025cc..7be95d7 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -219,6 +219,7 @@ asmlinkage void __do_softirq(void)
int cpu;
unsigned long old_flags = current->flags;
int max_restart = MAX_SOFTIRQ_RESTART;
+ int softirq_bit;

/*
* Mask out PF_MEMALLOC s current task context is borrowed for the
@@ -242,30 +243,30 @@ 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();

--
1.8.1.2.459.gbcd45b4.dirty


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