From: Daniel Bristot de Oliveira <firstname.lastname@example.org> To: Will Deacon <email@example.com>, Juri Lelli <firstname.lastname@example.org> Cc: Quentin Perret <email@example.com>, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, Catalin Marinas <email@example.com>, Marc Zyngier <firstname.lastname@example.org>, Greg Kroah-Hartman <email@example.com>, Peter Zijlstra <firstname.lastname@example.org>, Morten Rasmussen <email@example.com>, Qais Yousef <firstname.lastname@example.org>, Suren Baghdasaryan <email@example.com>, Tejun Heo <firstname.lastname@example.org>, Johannes Weiner <email@example.com>, Ingo Molnar <firstname.lastname@example.org>, Vincent Guittot <email@example.com>, "Rafael J. Wysocki" <firstname.lastname@example.org>, email@example.com Subject: Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED_DEADLINE Date: Fri, 21 May 2021 15:00:42 +0200 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <20210521103724.GA11680@willie-the-truck> On 5/21/21 12:37 PM, Will Deacon wrote: > Interesting, thanks. Thinking about this some more, it strikes me that with > these silly asymmetric systems there could be an interesting additional > problem with hotplug and deadline tasks. Imagine the following sequence of > events: > > 1. All online CPUs are 32-bit-capable > 2. sched_setattr() admits a 32-bit deadline task > 3. A 64-bit-only CPU is onlined At the point 3, the global scheduler assumption is broken. For instance, in a system with four CPUs and five ready 32-bit-capable tasks, when the fifth CPU as added, the working conserving rule is violated because the five highest priority thread are not running (only four are) :-(. So, at this point, for us to keep to the current behavior, the addition should be.. blocked? :-(( > 4. Some of the 32-bit-capable CPUs are offlined Assuming that point 3 does not exist (i.e., all CPUs are 32-bit-capable). At this point, we will have an increase in the pressure on the 32-bit-capable CPUs. This can also create bad effects for 64-bit tasks, as the "contended" 32-bit tasks will still be "queued" in a future time where they were supposed to be done (leaving time for the 64-bit tasks). > I wonder if we can get into a situation where we think we have enough > bandwidth available, but in reality the 32-bit task is in trouble because > it can't make use of the 64-bit-only CPU. I would have to think more, but there might be a case where this contended 32-bit tasks could cause deadline misses for the 64-bit too. > If so, then it seems to me that admission control is really just > "best-effort" for 32-bit deadline tasks on these systems because it's based > on a snapshot in time of the available resources. The admission test as is now is "best-effort" in the sense that it allows a workload higher than it could handle (it is necessary, but not sufficient AC). But it should not be considered "best-effort" because of violations in the working conserving property as a result of arbitrary affinities among tasks. Overall, we have been trying to close any "exception left" to this later case. I know, it is a complex situation, I am just trying to illustrate our concerns, because, in the near future we might have a scheduler that handles arbitrary affinity correctly. But that might require us to stick to an AC. The AC is something precious for us. (yeah, SP would not face this problem... now that I made progress on RV I can get back to it). -- Daniel _______________________________________________ linux-arm-kernel mailing list email@example.com http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2021-05-21 13:04 UTC|newest] Thread overview: 83+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-05-18 9:47 [PATCH v6 00/21] Add support for 32-bit tasks on asymmetric AArch32 systems Will Deacon 2021-05-18 9:47 ` [PATCH v6 01/21] arm64: cpuinfo: Split AArch32 registers out into a separate struct Will Deacon 2021-05-21 10:47 ` Catalin Marinas 2021-05-18 9:47 ` [PATCH v6 02/21] arm64: Allow mismatched 32-bit EL0 support Will Deacon 2021-05-21 10:25 ` Catalin Marinas 2021-05-24 12:05 ` Will Deacon 2021-05-24 13:49 ` Catalin Marinas 2021-05-21 10:41 ` Catalin Marinas 2021-05-24 12:09 ` Will Deacon 2021-05-24 13:46 ` Catalin Marinas 2021-05-21 15:22 ` Qais Yousef 2021-05-24 20:21 ` Will Deacon 2021-05-18 9:47 ` [PATCH v6 03/21] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched " Will Deacon 2021-05-21 10:47 ` Catalin Marinas 2021-05-18 9:47 ` [PATCH v6 04/21] arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs Will Deacon 2021-05-21 10:55 ` Catalin Marinas 2021-05-18 9:47 ` [PATCH v6 05/21] arm64: Advertise CPUs capable of running 32-bit applications in sysfs Will Deacon 2021-05-21 11:00 ` Catalin Marinas 2021-05-18 9:47 ` [PATCH v6 06/21] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection Will Deacon 2021-05-21 16:03 ` Peter Zijlstra 2021-05-24 12:17 ` Will Deacon 2021-05-18 9:47 ` [PATCH v6 07/21] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1 Will Deacon 2021-05-21 17:39 ` Qais Yousef 2021-05-24 20:21 ` Will Deacon 2021-05-18 9:47 ` [PATCH v6 08/21] cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus() Will Deacon 2021-05-21 16:25 ` Qais Yousef 2021-05-24 21:09 ` Will Deacon 2021-05-18 9:47 ` [PATCH v6 09/21] sched: Reject CPU affinity changes based on task_cpu_possible_mask() Will Deacon 2021-05-18 9:47 ` [PATCH v6 10/21] sched: Introduce task_struct::user_cpus_ptr to track requested affinity Will Deacon 2021-05-18 9:47 ` [PATCH v6 11/21] sched: Split the guts of sched_setaffinity() into a helper function Will Deacon 2021-05-21 16:41 ` Qais Yousef 2021-05-24 21:16 ` Will Deacon 2021-05-18 9:47 ` [PATCH v6 12/21] sched: Allow task CPU affinity to be restricted on asymmetric systems Will Deacon 2021-05-21 17:11 ` Qais Yousef 2021-05-24 21:43 ` Will Deacon 2021-05-18 9:47 ` [PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED_DEADLINE Will Deacon 2021-05-18 10:20 ` Quentin Perret 2021-05-18 10:28 ` Will Deacon 2021-05-18 10:48 ` Quentin Perret 2021-05-18 10:59 ` Will Deacon 2021-05-18 13:19 ` Quentin Perret 2021-05-20 9:13 ` Juri Lelli 2021-05-20 10:16 ` Will Deacon 2021-05-20 10:33 ` Quentin Perret 2021-05-20 12:38 ` Juri Lelli 2021-05-20 12:38 ` Daniel Bristot de Oliveira 2021-05-20 15:06 ` Dietmar Eggemann 2021-05-20 16:00 ` Daniel Bristot de Oliveira 2021-05-20 17:55 ` Dietmar Eggemann 2021-05-20 18:03 ` Will Deacon 2021-05-21 11:26 ` Dietmar Eggemann 2021-05-20 18:01 ` Will Deacon 2021-05-21 5:25 ` Juri Lelli 2021-05-21 8:15 ` Quentin Perret 2021-05-21 8:39 ` Juri Lelli 2021-05-21 10:37 ` Will Deacon 2021-05-21 11:23 ` Dietmar Eggemann 2021-05-21 13:02 ` Quentin Perret 2021-05-21 14:04 ` Juri Lelli 2021-05-21 17:47 ` Dietmar Eggemann 2021-05-21 13:00 ` Daniel Bristot de Oliveira [this message] 2021-05-21 13:12 ` Quentin Perret 2021-05-24 20:47 ` Will Deacon 2021-05-18 9:47 ` [PATCH v6 14/21] freezer: Add frozen_or_skipped() helper function Will Deacon 2021-05-18 9:47 ` [PATCH v6 15/21] sched: Defer wakeup in ttwu() for unschedulable frozen tasks Will Deacon 2021-05-18 9:47 ` [PATCH v6 16/21] arm64: Implement task_cpu_possible_mask() Will Deacon 2021-05-24 14:57 ` Catalin Marinas 2021-05-18 9:47 ` [PATCH v6 17/21] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0 Will Deacon 2021-05-24 15:02 ` Catalin Marinas 2021-05-18 9:47 ` [PATCH v6 18/21] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system Will Deacon 2021-05-24 15:46 ` Catalin Marinas 2021-05-24 20:32 ` Will Deacon 2021-05-25 9:43 ` Catalin Marinas 2021-05-18 9:47 ` [PATCH v6 19/21] arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0 Will Deacon 2021-05-24 15:47 ` Catalin Marinas 2021-05-18 9:47 ` [PATCH v6 20/21] arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores Will Deacon 2021-05-24 15:47 ` Catalin Marinas 2021-05-18 9:47 ` [PATCH v6 21/21] Documentation: arm64: describe asymmetric 32-bit support Will Deacon 2021-05-21 17:37 ` Qais Yousef 2021-05-24 21:46 ` Will Deacon 2021-05-24 16:22 ` Catalin Marinas 2021-05-21 17:45 ` [PATCH v6 00/21] Add support for 32-bit tasks on asymmetric AArch32 systems Qais Yousef 2021-05-24 22:08 ` Will Deacon
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --subject='Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED_DEADLINE' \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).