On Sun, Nov 17, 2013 at 01:55:10AM -0800, Joe Perches wrote:
> 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 <[email protected]>
> ---
> 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;
Perhaps using for_each_set_bit() would simplify that more?
> +
> + 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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