From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0C667C433F5 for ; Mon, 10 Oct 2022 15:42:22 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DFB896B0071; Mon, 10 Oct 2022 11:42:21 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id DAA396B0073; Mon, 10 Oct 2022 11:42:21 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C98DF6B0074; Mon, 10 Oct 2022 11:42:21 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id B744D6B0071 for ; Mon, 10 Oct 2022 11:42:21 -0400 (EDT) Received: from smtpin24.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 5B4921407E0 for ; Mon, 10 Oct 2022 15:42:21 +0000 (UTC) X-FDA: 80005456482.24.8DD59EF Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf08.hostedemail.com (Postfix) with ESMTP id 0479D160025 for ; Mon, 10 Oct 2022 15:42:19 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 6F03312FC; Mon, 10 Oct 2022 08:42:25 -0700 (PDT) Received: from wubuntu (unknown [10.57.35.2]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 391FD3F67D; Mon, 10 Oct 2022 08:42:18 -0700 (PDT) Date: Mon, 10 Oct 2022 16:42:16 +0100 From: Qais Yousef To: Hillf Danton Cc: John Stultz , LKML , linux-mm@kvack.org, Connor O'Brien , Peter Zijlstra , Steven Rostedt Subject: Re: [RFC PATCH v4 2/3] sched: Avoid placing RT threads on cores handling long softirqs Message-ID: <20221010154216.6mw7fszdaoajurvm@wubuntu> References: <20221003232033.3404802-3-jstultz@google.com> <20221004013611.1822-1-hdanton@sina.com> <20221005002149.1876-1-hdanton@sina.com> <20221005060155.1571-1-hdanton@sina.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20221005060155.1571-1-hdanton@sina.com> ARC-Authentication-Results: i=1; imf08.hostedemail.com; dkim=none; spf=pass (imf08.hostedemail.com: domain of qais.yousef@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=qais.yousef@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1665416540; a=rsa-sha256; cv=none; b=6K+9DHlF5PlmzIQlfx1OymhclJLKfpSX30PJHlANCZ86NExD+kkIitRmkeL5QuZBmHYl4B a1nHNIb6oQ1jVkxd4peQFSFDy9IUhGdHAFDnEi6lOlHQmqw2G7/tggUOvaS+ueSvFccgfU 6PUbOcfgrKfLjmxT+CxLUysajwi7/Xs= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1665416540; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=uhYoWsanSWRYY6DuGsO8F1bMw9DqwWBjND+JFLSvxNw=; b=qyyHkM57DBD+WBUJ0viFDxOG4ikAvJtclyNCCu/gOISkSROOV+nXA3rB25VGJBgWvDcPSU xpmlnPG/1ra4yGN29JrIUc2PwuR15P2HXyfSnviq9NJ4xdHgWzzn5bziGdRz+Y1SO0FHgB qMADzlqrlqqcNtE53w2xEh07vS94vG0= X-Rspamd-Queue-Id: 0479D160025 X-Rspam-User: Authentication-Results: imf08.hostedemail.com; dkim=none; spf=pass (imf08.hostedemail.com: domain of qais.yousef@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=qais.yousef@arm.com; dmarc=pass (policy=none) header.from=arm.com X-Rspamd-Server: rspam10 X-Stat-Signature: agz1rdtndipjs4r9xnbhhhs883izw833 X-HE-Tag: 1665416539-949237 X-Bogosity: Ham, tests=bogofilter, spamicity=0.017024, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 10/05/22 14:01, Hillf Danton wrote: > On 4 Oct 2022 18:13:52 -0700 John Stultz > > On Tue, Oct 4, 2022 at 5:22 PM Hillf Danton wrote: > > > On 3 Oct 2022 19:29:36 -0700 John Stultz > > > > > > > > Why would ksoftirqd preempt the rt task? > > > > > > > For example the kthread becomes sensitive to latency. > > > > Is it the case where > > the ksoftirqd thread is configured to run at higher rtprio? > > > Yes, you are right. I don't see a problem here. If a sys-admin configures their ksoftirqds to be a higher priority RT tasks than the audio threads, then they better know what they're doing :-) The issue at hand here is that the softirqs boundedness is hard to control. And the scheduling delays ensued are hard to deal with by any sys-admin. Networking has actually introduced some knobs to help control that - but the tricky bit of still being able to deliver high throughput networking while keeping the softirq bounded to minimize scheduling delays/latencies. I think even for PREEMPT_RT, high performance networking could be impacted to achieve the required low latency. See this paper which explores this duality: https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.702.7571&rep=rep1&type=pdf With WiFi 6 and 5G mobile networks, phones are actually expected to deliver multi-gigabit network throughputs. Cheers -- Qais Yousef