Commit c10d7367 authored Jan 10, 2013 by Eric Dumazet Committed by David S. Miller Jan 10, 2013

softirq: reduce latencies



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.

Using need_resched() as the only condition can trigger RCU stalls,
as we can keep BH disabled for too long.

I ran several benchmarks and got no significant difference in
throughput, but a very significant reduction of latencies (one order
of magnitude) :

In following bench, 200 antagonist "netperf -t TCP_RR" are started in
background, using all available cpus.

Then we start one "netperf -t TCP_RR", bound to the cpu handling the NIC
IRQ (hard+soft)

Before patch :

# netperf -H 7.7.7.84 -t TCP_RR -T2,2 -- -k
RT_LATENCY,MIN_LATENCY,MAX_LATENCY,P50_LATENCY,P90_LATENCY,P99_LATENCY,MEAN_LATENCY,STDDEV_LATENCY
MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET
to 7.7.7.84 () port 0 AF_INET : first burst 0 : cpu bind
RT_LATENCY=550110.424
MIN_LATENCY=146858
MAX_LATENCY=997109
P50_LATENCY=305000
P90_LATENCY=550000
P99_LATENCY=710000
MEAN_LATENCY=376989.12
STDDEV_LATENCY=184046.92

After patch :

# netperf -H 7.7.7.84 -t TCP_RR -T2,2 -- -k
RT_LATENCY,MIN_LATENCY,MAX_LATENCY,P50_LATENCY,P90_LATENCY,P99_LATENCY,MEAN_LATENCY,STDDEV_LATENCY
MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET
to 7.7.7.84 () port 0 AF_INET : first burst 0 : cpu bind
RT_LATENCY=40545.492
MIN_LATENCY=9834
MAX_LATENCY=78366
P50_LATENCY=33583
P90_LATENCY=59000
P99_LATENCY=69000
MEAN_LATENCY=38364.67
STDDEV_LATENCY=12865.26

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Miller <davem@davemloft.net>
Cc: Tom Herbert <therbert@google.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 1def9238

kernel/softirq.c

+9 −8

Original line number	Diff line number	Diff line
		@@ -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 msecs_to_jiffies(2)

		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 @@ asmlinkage void __do_softirq(void)
		local_irq_disable();

		pending = local_softirq_pending();
		if (pending && --max_restart)
		if (pending) {
		if (time_before(jiffies, end) && !need_resched())
		goto restart;

		if (pending)
		wakeup_softirqd();
		}

		lockdep_softirq_exit();