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 X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 01CF0C55185 for ; Wed, 22 Apr 2020 15:50:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CE34A2082E for ; Wed, 22 Apr 2020 15:50:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587570609; bh=ez7+ZsWjDbKtPTHLw+h/ZrnG00It65erPt+3scOnxEI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=aISpwQVcq1N113rGFFr+ahOV2azWT1HnIGYcOR9ZLWpGJgiV1oO3KAT+fnYzpAZYx C4nGuY5rU59BUIIy/4qFJRphPznBuRLr3PgtThT9gINdy/hKEXnXLuVsyyiuua0ren LznEeAykocfxEO3jdyA5JYOSdVCll3EPO/0Cs7D4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726626AbgDVPuJ (ORCPT ); Wed, 22 Apr 2020 11:50:09 -0400 Received: from mail.kernel.org ([198.145.29.99]:55092 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726579AbgDVPuI (ORCPT ); Wed, 22 Apr 2020 11:50:08 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 0F1CB20767; Wed, 22 Apr 2020 15:50:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587570608; bh=ez7+ZsWjDbKtPTHLw+h/ZrnG00It65erPt+3scOnxEI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=N319B63a8bPYR1tPct/qnNKws4vGPRf7xGiJPSfc5mpsTEdUfvKZZ0svx6z4r/7ha UncFjsX/iCEj+1bpXMVvqQirWXHX1XPIIqD95XzMVmsOj9yqrQ/zgBJWucHXnVU2An TdnzeGHQtp+e9NqC810NhqaKAUBIIoOxNr8JSBCw= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 9A3FC35203BC; Wed, 22 Apr 2020 08:50:06 -0700 (PDT) Date: Wed, 22 Apr 2020 08:50:06 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, rostedt@goodmis.org, qais.yousef@arm.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, bsegall@google.com, mgorman@suse.de, airlied@redhat.com, alexander.deucher@amd.com, awalls@md.metrocast.net, axboe@kernel.dk, broonie@kernel.org, daniel.lezcano@linaro.org, gregkh@linuxfoundation.org, hannes@cmpxchg.org, herbert@gondor.apana.org.au, hverkuil@xs4all.nl, john.stultz@linaro.org, nico@fluxnic.net, rafael.j.wysocki@intel.com, rmk+kernel@arm.linux.org.uk, sudeep.holla@arm.com, ulf.hansson@linaro.org, wim@linux-watchdog.org Subject: Re: [PATCH 01/23] sched: Provide sched_set_fifo() Message-ID: <20200422155006.GR17661@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200422112719.826676174@infradead.org> <20200422112831.266499893@infradead.org> <20200422131138.GL17661@paulmck-ThinkPad-P72> <20200422132648.GJ20730@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200422132648.GJ20730@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 22, 2020 at 03:26:48PM +0200, Peter Zijlstra wrote: > On Wed, Apr 22, 2020 at 06:11:38AM -0700, Paul E. McKenney wrote: > > On Wed, Apr 22, 2020 at 01:27:20PM +0200, Peter Zijlstra wrote: > > > SCHED_FIFO (or any static priority scheduler) is a broken scheduler > > > model; it is fundamentally incapable of resource management, the one > > > thing an OS is actually supposed to do. > > > > > > It is impossible to compose static priority workloads. One cannot take > > > two well designed and functional static priority workloads and mash > > > them together and still expect them to work. > > > > > > Therefore it doesn't make sense to expose the priority field; the > > > kernel is fundamentally incapable of setting a sensible value, it > > > needs systems knowledge that it doesn't have. > > > > > > Take away sched_setschedule() / sched_setattr() from modules and > > > replace them with: > > > > > > - sched_set_fifo(p); create a FIFO task (at prio 50) > > > - sched_set_fifo_low(p); create a task higher than NORMAL, > > > which ends up being a FIFO task at prio 1. > > > - sched_set_normal(p, nice); (re)set the task to normal > > > > > > This stops the proliferation of randomly chosen, and irrelevant, FIFO > > > priorities that dont't really mean anything anyway. > > > > > > The system administrator/integrator, whoever has insight into the > > > actual system design and requirements (userspace) can set-up > > > appropriate priorities if and when needed. > > > > The sched_setscheduler_nocheck() calls in rcu_spawn_gp_kthread(), > > rcu_cpu_kthread_setup(), and rcu_spawn_one_boost_kthread() all stay as > > is because they all use the rcutree.kthread_prio boot parameter, which is > > set at boot time by the system administrator (or {who,what}ever, correct? > > Correct, also they are not modular afaict, so they escaped the dance ;-) Indeed, an extreme form of insanity would be required to try to make core RCU be a module. Not that such a form of insanity is a bad thing in and of itself, but it might best be directed towards less futile ventures. ;-) Thanx, Paul