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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_2 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 43ACEC433E0 for ; Fri, 7 Aug 2020 14:16:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2C21620866 for ; Fri, 7 Aug 2020 14:16:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726155AbgHGOQr (ORCPT ); Fri, 7 Aug 2020 10:16:47 -0400 Received: from mail.santannapisa.it ([193.205.80.98]:52489 "EHLO mail.santannapisa.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725815AbgHGOQr (ORCPT ); Fri, 7 Aug 2020 10:16:47 -0400 Received: from [94.37.192.58] (account l.abeni@santannapisa.it HELO nowhere) by santannapisa.it (CommuniGate Pro SMTP 6.1.11) with ESMTPSA id 150296581; Fri, 07 Aug 2020 15:16:43 +0200 Date: Fri, 7 Aug 2020 15:16:32 +0200 From: luca abeni To: Juri Lelli Cc: peterz@infradead.org, mingo@redhat.com, rostedt@goodmis.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, tommaso.cucinotta@santannapisa.it, alessio.balsini@gmail.com, bristot@redhat.com, dietmar.eggemann@arm.com, linux-rt-users@vger.kernel.org, mtosatti@redhat.com, williams@redhat.com, valentin.schneider@arm.com Subject: Re: [RFC PATCH v2 0/6] SCHED_DEADLINE server infrastructure Message-ID: <20200807151632.36dc6200@nowhere> In-Reply-To: <20200807095051.385985-1-juri.lelli@redhat.com> References: <20200807095051.385985-1-juri.lelli@redhat.com> Organization: Scuola Superiore S.Anna X-Mailer: Claws Mail 3.17.5 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org Hi Juri, thanks for sharing the v2 patchset! In the next days I'll have a look at it, and try some tests... In the meanwhile, I have some questions/comments after a first quick look. If I understand well, the patchset does not apply deadline servers to FIFO and RR tasks, right? How does this patchset interact with RT throttling? If I understand well, patch 6/6 does something like "use deadline servers for SCHED_OTHER only if FIFO/RR tasks risk to starve SCHED_OTHER tasks"... Right? I understand this is because you do not want to delay RT tasks if they are not starving other tasks. But then, maybe what you want is not deadline-based scheduling. Maybe a reservation-based scheduler based on fixed priorities is what you want? (with SCHED_DEADLINE, you could provide exact performance guarantees to SCHED_OTHER tasks, but I suspect patch 6/6 breaks these guarantees?) Thanks, Luca On Fri, 7 Aug 2020 11:50:45 +0200 Juri Lelli wrote: > Hi, > > This is RFC v2 of Peter's SCHED_DEADLINE server infrastructure > implementation [1]. > > SCHED_DEADLINE servers can help fixing starvation issues of low > priority tasks (e.g., SCHED_OTHER) when higher priority tasks > monopolize CPU cycles. Today we have RT Throttling; DEADLINE servers > should be able to replace and improve that. > > I rebased Peter's patches (adding changelogs where needed) on > tip/sched/core as of today and incorporated fixes to issues discussed > during RFC v1. Current set seems to even boot on real HW! :-) > > While playing with RFC v1 set (and discussing it further offline with > Daniel) it has emerged the need to slightly change the behavior. Patch > 6/6 is a (cumbersome?) attempt to show what's probably needed. > The problem with "original" implementation is that FIFO tasks might > suffer preemption from NORMAL even when spare CPU cycles are > available. In fact, fair deadline server is enqueued right away when > NORMAL tasks wake up and they are first scheduled by the server, thus > potentially preempting a well behaving FIFO task. This is of course > not ideal. So, in patch 6/6 I propose to use some kind of starvation > monitor/ watchdog that delays enqueuing of deadline servers to the > point when fair tasks might start to actually suffer from starvation > (just randomly picked HZ/2 for now). One problem I already see with > the current implementation is that it adds overhead to fair paths, so > I'm pretty sure there are better ways to implement the idea (e.g., > Daniel already suggested using a starvation monitor kthread sort of > thing). > > Receiving comments and suggestions is the sole purpose of this posting > at this stage. Hopefully we can further discuss the idea at Plumbers > in a few weeks. So, please don't focus too much into actual > implementation (which I plan to revise anyway after I'm back from pto > :), but try to see if this might actually fly. The feature seems to > be very much needed. > > Thanks! > > Juri > > 1 - > https://lore.kernel.org/lkml/20190726145409.947503076@infradead.org/ > > Juri Lelli (1): > sched/fair: Implement starvation monitor > > Peter Zijlstra (5): > sched: Unify runtime accounting across classes > sched/deadline: Collect sched_dl_entity initialization > sched/deadline: Move bandwidth accounting into > {en,de}queue_dl_entity sched/deadline: Introduce deadline servers > sched/fair: Add trivial fair server > > include/linux/sched.h | 28 ++- > kernel/sched/core.c | 23 +- > kernel/sched/deadline.c | 483 > ++++++++++++++++++++++++--------------- kernel/sched/fair.c | > 136 ++++++++++- kernel/sched/rt.c | 17 +- > kernel/sched/sched.h | 50 +++- > kernel/sched/stop_task.c | 16 +- > 7 files changed, 522 insertions(+), 231 deletions(-) >