linux-kernel - [PATCH 1/3] softirq: Use ffs in __do

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Date:	Sun, 17 Nov 2013 01:55:10 -0800
From:	Joe Perches <joe@...ches.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] softirq: Use ffs in __do_softirq

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@...ches.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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