[PATCH net-next] softirq: reduce latencies
From: Eric Dumazet <hidden>
Date: 2013-01-03 12:28:59
Also in:
lkml
Subsystem:
the rest · Maintainer:
Linus Torvalds
From: Eric Dumazet <edumazet@google.com> In various network workloads, __do_softirq() latencies can be up to 20 ms if HZ=1000, and 200 ms if HZ=100. This is because we iterate 10 times in the softirq dispatcher, and some actions can consume a lot of cycles. This patch changes the fallback to ksoftirqd condition to : - A time limit of 2 ms. - need_resched() being set on current task When one of this condition is met, we wakeup ksoftirqd for further softirq processing if we still have pending softirqs. I ran several benchmarks and got no significant difference in throughput, but a very significant reduction of maximum latencies. Signed-off-by: Eric Dumazet <edumazet@google.com> Cc: David Miller <davem@davemloft.net> Cc: Tom Herbert <redacted> --- kernel/softirq.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-)
diff --git a/kernel/softirq.c b/kernel/softirq.c
index ed567ba..64d61ea 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c@@ -195,21 +195,21 @@ void local_bh_enable_ip(unsigned long ip) EXPORT_SYMBOL(local_bh_enable_ip); /* - * We restart softirq processing MAX_SOFTIRQ_RESTART times, - * and we fall back to softirqd after that. + * We restart softirq processing for at most 2 ms, + * and if need_resched() is not set. * - * This number has been established via experimentation. + * These limits have been established via experimentation. * The two things to balance is latency against fairness - * we want to handle softirqs as soon as possible, but they * should not be able to lock up the box. */ -#define MAX_SOFTIRQ_RESTART 10 +#define MAX_SOFTIRQ_TIME min(1, (2*HZ/1000)) asmlinkage void __do_softirq(void) { struct softirq_action *h; __u32 pending; - int max_restart = MAX_SOFTIRQ_RESTART; + unsigned long end = jiffies + MAX_SOFTIRQ_TIME; int cpu; unsigned long old_flags = current->flags;
@@ -264,11 +264,12 @@ restart: local_irq_disable(); pending = local_softirq_pending(); - if (pending && --max_restart) - goto restart; + if (pending) { + if (time_before(jiffies, end) && !need_resched()) + goto restart; - if (pending) wakeup_softirqd(); + } lockdep_softirq_exit();