From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH net-next] softirq: reduce latencies Date: Thu, 3 Jan 2013 12:46:08 -0800 Message-ID: <20130103124608.136fd65b.akpm@linux-foundation.org> References: <1357216132.21409.24107.camel@edumazet-glaptop> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: David Miller , netdev , "linux-kernel@vger.kernel.org" , Tom Herbert To: Eric Dumazet Return-path: In-Reply-To: <1357216132.21409.24107.camel@edumazet-glaptop> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Thu, 03 Jan 2013 04:28:52 -0800 Eric Dumazet wrote: > From: Eric Dumazet > > 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. hm, where did that "20 ms" come from? What caused it? Is it simply the case that you happened to have actions which consume 2ms if HZ=1000 and 20ms if HZ=100? > 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. Do we need both tests? The need_resched() test alone might be sufficient? With this change, there is a possibility that a rapidly-rescheduling task will cause softirq starvation? Can this change cause worsened latencies in some situations? Say there are a large number of short-running actions queued. Presently we'll dispatch ten of them and return. With this change we'll dispatch many more of them - however many consume 2ms. So worst-case latency increases from "10 * not-much" to "2 ms".