From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752098AbeAWSYa (ORCPT ); Tue, 23 Jan 2018 13:24:30 -0500 Received: from shards.monkeyblade.net ([184.105.139.130]:57284 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751547AbeAWSY3 (ORCPT ); Tue, 23 Jan 2018 13:24:29 -0500 Date: Tue, 23 Jan 2018 13:24:24 -0500 (EST) Message-Id: <20180123.132424.1035340800864767853.davem@davemloft.net> To: torvalds@linux-foundation.org Cc: pabeni@redhat.com, frederic@kernel.org, linux-kernel@vger.kernel.org, alexander.levin@verizon.com, peterz@infradead.org, mchehab@s-opensource.com, hannes@stressinduktion.org, paulmck@linux.vnet.ibm.com, wanpeng.li@hotmail.com, dima@arista.com, tglx@linutronix.de, akpm@linux-foundation.org, rrendec@arista.com, mingo@kernel.org, sgruszka@redhat.com, riel@redhat.com, edumazet@google.com Subject: Re: [RFC PATCH 0/4] softirq: Per vector threading v3 From: David Miller In-Reply-To: References: <20180123.112201.1263563609292212852.davem@davemloft.net> <1516726652.2554.58.camel@redhat.com> X-Mailer: Mew version 6.7 on Emacs 25.3 / Mule 6.0 (HANACHIRUSATO) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Linus Torvalds Date: Tue, 23 Jan 2018 09:42:32 -0800 > But I wonder if the test triggers the "lets run lots of workqueue > threads", and then the single-threaded user space just gets blown out > of the water by many kernel threads. Each thread gets its own "fair" > amount of CPU, but.. If a single cpu's softirq deferral can end up running on multiple workqueue threads, indeed that's a serious problem. So if we're in a workqueue and it does a: schedule_work_on(this_cpu, currently_executing_work); it'll potentially make a new thread? That's exactly the code path that will get exercised during a UDP flood the way that vector_work_func() is implemented.