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Message-ID: <20140410115706.662fb5e7@annuminas.surriel.com> Date: Thu, 10 Apr 2014 11:57:06 -0400 From: Rik van Riel <riel@...hat.com> To: linux-kernel@...r.kernel.org Cc: netdev@...r.kernel.org, Jiri Benc <jbenc@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, David Miller <davem@...emloft.net>, Frederic Weisbecker <fweisbec@...il.com> Subject: [PATCH] softirq: punt to ksoftirqd if __do_softirq recently looped Jiri noticed that netperf throughput had gotten worse in recent years, for smaller message sizes. In the past, ksoftirqd would take around 80% of a CPU, and netserver would take around 100% of another CPU. On current kernels, sometimes all the softirq processing is done in the context of the netperf process, which can result in as much as a 50% performance drop, due to netserver spending all its CPU time "delivering" packets to a socket it rarely empties, and dropping the packets on the floor as a result. This seems silly in an age where even cell phones are multi-core, and we could simply let the ksoftirqd thread handle the softirq load, so the scheduler can migrate the userspace task to another CPU. This patch accomplishes that in a very simplistic way. The code remembers when __do_softirq last looped, and will punt softirq handling to ksoftirqd if another softirq happens in the same jiffie. Netperf results: without patch with patch UDP_STREAM 1472 957.17 / 954.18 957.15 / 951.73 UDP_STREAM 978 936.85 / 930.06 936.84 / 927.63 UDP_STREAM 466 875.98 / 865.62 875.98 / 868.65 UDP_STREAM 210 760.88 / 748.70 760.88 / 748.61 UDP_STREAM 82 554.06 / 329.96 554.06 / 505.95 unstable ^^^^^^ UDP_STREAM 18 158.99 / 108.95 160.73 / 112.68 Signed-off-by: Rik van Riel <riel@...hat.com> Cc: Frederic Weisbecker <fweisbec@...il.com> Cc: David Miller <davem@...emloft.net> Cc: Thomas Gleixner <tglx@...utronix.de> Cc: Peter Zijlstra <peterz@...radead.org> Tested-by: Jiri Benc <jbenc@...hat.com> Reported-by: Jiri Benc <jbenc@...hat.com> --- kernel/softirq.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/kernel/softirq.c b/kernel/softirq.c index 787b3a0..020be2f 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -56,6 +56,7 @@ EXPORT_SYMBOL(irq_stat); static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp; DEFINE_PER_CPU(struct task_struct *, ksoftirqd); +DEFINE_PER_CPU(unsigned long, softirq_looped); char *softirq_to_name[NR_SOFTIRQS] = { "HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "BLOCK_IOPOLL", @@ -271,6 +272,9 @@ asmlinkage void __do_softirq(void) pending = local_softirq_pending(); if (pending) { + /* Still busy? Remember this for invoke_softirq() below... */ + this_cpu_write(softirq_looped, jiffies); + if (time_before(jiffies, end) && !need_resched() && --max_restart) goto restart; @@ -330,7 +334,11 @@ void irq_enter(void) static inline void invoke_softirq(void) { - if (!force_irqthreads) { + /* + * If force_irqthreads is set, or if we looped in __do_softirq this + * jiffie, punt to ksoftirqd to prevent userland starvation. + */ + if (!force_irqthreads && this_cpu_read(softirq_looped) != jiffies) { /* * We can safely execute softirq on the current stack if * it is the irq stack, because it should be near empty -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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