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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 AC3DEC433E1 for ; Thu, 11 Jun 2020 10:24:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8D7E220760 for ; Thu, 11 Jun 2020 10:24:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727782AbgFKKYT (ORCPT ); Thu, 11 Jun 2020 06:24:19 -0400 Received: from foss.arm.com ([217.140.110.172]:50022 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726693AbgFKKYO (ORCPT ); Thu, 11 Jun 2020 06:24:14 -0400 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 6F2CD31B; Thu, 11 Jun 2020 03:24:13 -0700 (PDT) Received: from e107158-lin.cambridge.arm.com (e107158-lin.cambridge.arm.com [10.1.195.21]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B331C3F73D; Thu, 11 Jun 2020 03:24:10 -0700 (PDT) Date: Thu, 11 Jun 2020 11:24:08 +0100 From: Qais Yousef To: Vincent Guittot Cc: Mel Gorman , Patrick Bellasi , Dietmar Eggemann , Peter Zijlstra , Ingo Molnar , Randy Dunlap , Jonathan Corbet , Juri Lelli , Steven Rostedt , Ben Segall , Luis Chamberlain , Kees Cook , Iurii Zaikin , Quentin Perret , Valentin Schneider , Pavan Kondeti , linux-doc@vger.kernel.org, linux-kernel , linux-fs Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value Message-ID: <20200611102407.vhy3zjexrhorx753@e107158-lin.cambridge.arm.com> References: <20200528161112.GI2483@worktop.programming.kicks-ass.net> <20200529100806.GA3070@suse.de> <87v9k84knx.derkling@matbug.net> <20200603101022.GG3070@suse.de> <20200603165200.v2ypeagziht7kxdw@e107158-lin.cambridge.arm.com> <20200608123102.6sdhdhit7lac5cfl@e107158-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20171215 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 06/09/20 19:10, Vincent Guittot wrote: > On Mon, 8 Jun 2020 at 14:31, Qais Yousef wrote: > > > > On 06/04/20 14:14, Vincent Guittot wrote: > > > > [...] > > > > > I have tried your patch and I don't see any difference compared to > > > previous tests. Let me give you more details of my setup: > > > I create 3 levels of cgroups and usually run the tests in the 4 levels > > > (which includes root). The result above are for the root level > > > > > > But I see a difference at other levels: > > > > > > root level 1 level 2 level 3 > > > > > > /w patch uclamp disable 50097 46615 43806 41078 > > > tip uclamp enable 48706(-2.78%) 45583(-2.21%) 42851(-2.18%) > > > 40313(-1.86%) > > > /w patch uclamp enable 48882(-2.43%) 45774(-1.80%) 43108(-1.59%) > > > 40667(-1.00%) > > > > > > Whereas tip with uclamp stays around 2% behind tip without uclamp, the > > > diff of uclamp with your patch tends to decrease when we increase the > > > number of level > > > > So I did try to dig more into this, but I think it's either not a good > > reproducer or what we're observing here is uArch level latencies caused by the > > new code that seem to produce a bigger knock on effect than what they really > > are. > > > > First, CONFIG_FAIR_GROUP_SCHED is 'expensive', for some definition of > > expensive.. > > yes, enabling CONFIG_FAIR_GROUP_SCHED adds an overhead > > > > > *** uclamp disabled/fair group enabled *** > > > > # Executed 50000 pipe operations between two threads > > > > Total time: 0.958 [sec] > > > > 19.177100 usecs/op > > 52145 ops/sec > > > > *** uclamp disabled/fair group disabled *** > > > > # Executed 50000 pipe operations between two threads > > Total time: 0.808 [sec] > > > > 16.176200 usecs/op > > 61819 ops/sec > > > > So there's a 15.6% drop in ops/sec when enabling this option. I think it's good > > to look at the absolutely number of usecs/op, Fair group adds around > > 3 usecs/op. > > > > I dropped FAIR_GROUP_SCHED from my config to eliminate this overhead and focus > > on solely on uclamp overhead. > > Have you checked that both tests run at the root level ? I haven't actively moved tasks to cgroups. As I said that snippet was particularly bad and I didn't see that level of nesting in every call. > Your function-graph log below shows several calls to > update_cfs_group() which means that your trace below has not been made > at root level but most probably at the 3rd level and I wonder if you > used the same setup for running the benchmark above. This could > explain such huge difference because I don't have such difference on > my platform but more around 2% What promoted me to look at this is when you reported that even without uclamp the nested cgroup showed a drop at each level. I was just trying to understand how both affect the hot path in hope to understand the root cause of uclamp overhead. > > For uclamp disable/fair group enable/ function graph enable : 47994ops/sec > For uclamp disable/fair group disable/ function graph enable : 49107ops/sec > > > > > With uclamp enabled but no fair group I get > > > > *** uclamp enabled/fair group disabled *** > > > > # Executed 50000 pipe operations between two threads > > Total time: 0.856 [sec] > > > > 17.125740 usecs/op > > 58391 ops/sec > > > > The drop is 5.5% in ops/sec. Or 1 usecs/op. > > > > I don't know what's the expectation here. 1 us could be a lot, but I don't > > think we expect the new code to take more than few 100s of ns anyway. If you > > add potential caching effects, reaching 1 us wouldn't be that hard. > > > > Note that in my runs I chose performance governor and use `taskset 0x2` to > > You might want to set 2 CPUs in your cpumask instead of 1 in order to > have 1 CPU for each thread I did try that but it didn't seem to change the number. I think the 2 tasks interleave so running in 2 CPUs doesn't change the result. But to ease ftrace capture, it's easier to monitor a single cpu. > > > force running on a big core to make sure the runs are repeatable. > > I also use performance governor but don't pinned tasks because I use smp. Is your arm platform SMP? > > > > > On Juno-r2 I managed to scrap most of the 1 us with the below patch. It seems > > there was weird branching behavior that affects the I$ in my case. It'd be good > > to try it out to see if it makes a difference for you. > > The perf are slightly worse on my setup: > For uclamp enable/fair group disable/ function graph enable : 48413ops/sec > with patch below : 47804os/sec I am not sure if the new code could just introduce worse cache performance in a platform dependent way. The evidences I have so far point in this direction. > > > > > The I$ effect is my best educated guess. Perf doesn't catch this path and > > I couldn't convince it to look at cache and branch misses between 2 specific > > points. > > > > Other subtle code shuffling did have weird effect on the result too. One worthy > > one is making uclamp_rq_dec() noinline gains back ~400 ns. Making > > uclamp_rq_inc() noinline *too* cancels this gain out :-/ > > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 0464569f26a7..0835ee20a3c7 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -1071,13 +1071,11 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p, > > > > static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p) > > { > > - enum uclamp_id clamp_id; > > - > > if (unlikely(!p->sched_class->uclamp_enabled)) > > return; > > > > - for_each_clamp_id(clamp_id) > > - uclamp_rq_inc_id(rq, p, clamp_id); > > + uclamp_rq_inc_id(rq, p, UCLAMP_MIN); > > + uclamp_rq_inc_id(rq, p, UCLAMP_MAX); > > > > /* Reset clamp idle holding when there is one RUNNABLE task */ > > if (rq->uclamp_flags & UCLAMP_FLAG_IDLE) > > @@ -1086,13 +1084,11 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p) > > > > static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p) > > { > > - enum uclamp_id clamp_id; > > - > > if (unlikely(!p->sched_class->uclamp_enabled)) > > return; > > > > - for_each_clamp_id(clamp_id) > > - uclamp_rq_dec_id(rq, p, clamp_id); > > + uclamp_rq_dec_id(rq, p, UCLAMP_MIN); > > + uclamp_rq_dec_id(rq, p, UCLAMP_MAX); > > } > > > > static inline void > > > > > > FWIW I fail to see activate/deactivate_task in perf record. They don't show up > > on the list which means this micro benchmark doesn't stress them as Mel's test > > does. > > Strange because I have been able to trace them. On your arm platform? I can certainly see them on x86. Thanks -- Qais Yousef