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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E26FCC433F5 for ; Tue, 26 Apr 2022 10:07:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348396AbiDZKKM (ORCPT ); Tue, 26 Apr 2022 06:10:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44566 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1348745AbiDZKJN (ORCPT ); Tue, 26 Apr 2022 06:09:13 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 2CB3B1EADD0 for ; Tue, 26 Apr 2022 02:31:07 -0700 (PDT) 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 DC73A23A; Tue, 26 Apr 2022 02:31:06 -0700 (PDT) Received: from airbuntu (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4BB4F3F73B; Tue, 26 Apr 2022 02:31:05 -0700 (PDT) Date: Tue, 26 Apr 2022 10:30:56 +0100 From: Qais Yousef To: Vincent Guittot Cc: Xuewen Yan , Xuewen Yan , dietmar.eggemann@arm.com, lukasz.luba@arm.com, rafael@kernel.org, viresh.kumar@linaro.org, mingo@redhat.com, peterz@infradead.org, rostedt@goodmis.org, linux-kernel@vger.kernel.org, di.shen@unisoc.com Subject: Re: [PATCH] sched: Take thermal pressure into account when determine rt fits capacity Message-ID: <20220426093056.uxnsz4tv4vhvbipe@airbuntu> References: <20220407051932.4071-1-xuewen.yan@unisoc.com> <20220420135127.o7ttm5tddwvwrp2a@airbuntu> <20220421161509.asz25zmh25eurgrk@airbuntu> <20220425161209.ydugtrs3b7gyy3kk@airbuntu> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/26/22 10:09, Vincent Guittot wrote: > On Tue, 26 Apr 2022 at 04:07, Xuewen Yan wrote: > > > > On Tue, Apr 26, 2022 at 12:12 AM Qais Yousef wrote: > > > > > > On 04/25/22 09:31, Xuewen Yan wrote: > > > > On Fri, Apr 22, 2022 at 12:15 AM Qais Yousef wrote: > > > > > Is it okay to share what the capacities of the littles, mediums and bigs on > > > > > your system? And how they change under worst case scenario thermal pressure? > > > > > Only IF you have these numbers handy :-) > > > > > > > > Okay, the little/mid/big cpu scale capacity is 350/930/1024, but the > > > > cpu frequency point is discrete, the big core's high freq point may is > > > > just a few more than the mid core's highest. > > > > In this case, once the thermal decrease the scaling_max_freq, the > > > > maximum frequency of the large core is easily lower than that of the > > > > medium core. > > > > Of course, the corner case is due to the frequency design of the soc > > > > and our thermal algorithm. > > > > > > Okay, thanks for the info! > > > > > > > > > > > > > > > > > Is it actually an indication of a potential other problem if you swing into > > > > > capacity inversion in the bigs that often? I've seen a lot of systems where the > > > > > difference between the meds and bigs is small. But frequent inversion could be > > > > > suspicious still. > > > > > > > > > > Do the littles and the mediums experience any significant thermal pressure too? > > > > > > > > In our platform, it's not. > > > > > > Good. > > > > > > > > It doesn't seem it'll cause a significant error, but still it seems to me this > > > > > function wants the original capacity passed to it. > > > > > > > > > > There are similar questions to be asked since you modify sg_cpu->max. Every > > > > > user needs to be audited if they're fine with this change or not. > > > > > > > > > > I'm not sure still what we are achieving here. You want to force schedutil not > > > > > to request higher frequencies if thermal pressure is high? Should schedutil > > > > > actually care? Shouldn't the cpufreq driver reject this request and pick the > > > > > next best thing if it can't satisfy it? I could be missing something, I haven't > > > > > looked that hard tbh :-) > > > > > > > > I changed this just want to make it more responsive to the real > > > > capacity of the cpu, if it will cause other problems, maybe it would > > > > be better not to change it.:) > > > > > > There are others who can give you a better opinion. But AFAICS we're not fixing > > > anything but risking breaking other things. So I vote for not to change it :) > > > > > > > > It depends on the severity of the problem. The simplest thing I can suggest is > > > > > to check if the cpu is in capacity inversion state, and if it is, then make > > > > > rt_task_fits_capacity() return false always. > > > > > > > > > > If we need a generic solution to handle thermal pressure omitting OPPs, then > > > > > the search needs to become more complex. The proposal in this patch is not > > > > > adequate because tasks that want to run at capacity_orig_of(cpu) will wrongly > > > > > omit some cpus because of any tiny thermal pressure. For example if the > > > > > capacity_orig_of(medium_cpu) = 700, and uclamp_min for RT is set to 700, then > > > > > any small thermal pressure on mediums will cause these tasks to run on big cpus > > > > > only, which is not what we want. Especially if these big cpus can end up in > > > > > capacity inversion later ;-) > > > > > > > > > > So if we want to handle this case, then we need to ensure the search returns > > > > > false only if > > > > > > > > > > 1. Thermal pressure results in real OPP to be omitted. > > > > > 2. Another CPU that can provide this performance level is available. > > > > > > > > > > Otherwise we should still fit it on this CPU because it'll give us the closest > > > > > thing to what was requested. > > > > > > > > > > I can think of 2 ways to implement this, but none of them seem particularly > > > > > pretty :-/ > > > > > > > > Maybe as Lukasz Luba said: > > > > > > > > https://lore.kernel.org/all/ae98a861-8945-e630-8d4c-8112723d1007@arm.com/ > > > > > > > > > Let's meet in the middle: > > > > > 1) use the thermal PELT signal in RT: > > > > > capacity = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu)) > > > > > 2) introduce a more configurable thermal_pressure shifter instead > > > > > 'sched_thermal_decay_shift', which would allow not only to make the > > > > > decaying longer, but also shorter when the platform already might do > > > > > that, to not cause too much traffic. > > > > > > > > But even if this is changed, there will still be the same problem, I > > > > look forward to Lukasz's patch:) > > > > > > This will not address my concern unless I missed something. > > > > > > The best (simplest) way forward IMHO is to introduce a new function > > > > > > bool cpu_in_capacity_inversion(int cpu); > > > > > > (feel free to pick another name) which will detect the scenario you're in. You > > > can use this function then in rt_task_fits_capacity() > > > > > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > > > index a32c46889af8..d48811a7e956 100644 > > > --- a/kernel/sched/rt.c > > > +++ b/kernel/sched/rt.c > > > @@ -462,6 +462,9 @@ static inline bool rt_task_fits_capacity(struct task_struct *p, int cpu) > > > if (!static_branch_unlikely(&sched_asym_cpucapacity)) > > > return true; > > > > > > + if (cpu_in_capacity_inversion(cpu)) > > > + return false; > > > + > > > min_cap = uclamp_eff_value(p, UCLAMP_MIN); > > > max_cap = uclamp_eff_value(p, UCLAMP_MAX); > > > > > > You'll probably need to do something similar in dl_task_fits_capacity(). > > > > > > This might be a bit aggressive though as we'll steer away all RT tasks from > > > this CPU (as long as there's another CPU that can fit it). I need to think more > > > about it. But we could do something like this too > > > > > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > > > index a32c46889af8..f2a34946a7ab 100644 > > > --- a/kernel/sched/rt.c > > > +++ b/kernel/sched/rt.c > > > @@ -462,11 +462,14 @@ static inline bool rt_task_fits_capacity(struct task_struct *p, int cpu) > > > if (!static_branch_unlikely(&sched_asym_cpucapacity)) > > > return true; > > > > > > + cpu_cap = capacity_orig_of(cpu); > > > + > > > + if (cpu_in_capacity_inversion(cpu)) > > > > It's a good idea, but as you said, in mainline, the > > sysctl_sched_uclamp_util_min_rt_default is always 1024, > > Maybe it's better to add it to the judgment? > > > > + if (sysctl_sched_uclamp_util_min_rt_default != > > SCHED_CAPACITY_SCALE && cpu_in_capacity_inversion(cpu)) > > > > > + cpu_cap -= thermal_load_avg(cpu_rq(cpu)); > > > > Why use thermal_load_avg? If thermal is always in effect，the > > thermal_load_avg would get bigger and bigger, as a result, the cpu_cap > > maybe smaller than (capacity_orig - thermal_pressure). > > For a fixed thermal_pressure(), thermal_load_avg() will not be higher > than thermal_pressure() but will increase to reach thermal_pressure() > > In the current implementation for sched_asym_cpucapacity topology, we > do a 1st iteration trying to find a cpu that fits a task's capacity > but if it fails, we run a normal cpupri_find that doesn't care about > capacity. > > Do I understand correctly that in your case you would like to run > a 1st iteration that takes into account capacity_orig_of(cpu) - > thermal_load_avg(cpu_rq(cpu)) > If it fails run another iteration only with capacity_orig_of(cpu) > and finally tries without capacity constraint Wouldn't this be expensive to have 3 loops? That was my other suggestion but wasn't sure the complexity was worth it. So I suggested handling the capacity inversion case only. Thanks -- Qais Yousef