From: Will Deacon <email@example.com> To: Daniel Bristot de Oliveira <firstname.lastname@example.org> Cc: Juri Lelli <email@example.com>, Quentin Perret <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com, Catalin Marinas <firstname.lastname@example.org>, Marc Zyngier <email@example.com>, Greg Kroah-Hartman <firstname.lastname@example.org>, Peter Zijlstra <email@example.com>, Morten Rasmussen <firstname.lastname@example.org>, Qais Yousef <email@example.com>, Suren Baghdasaryan <firstname.lastname@example.org>, Tejun Heo <email@example.com>, Johannes Weiner <firstname.lastname@example.org>, Ingo Molnar <email@example.com>, Vincent Guittot <firstname.lastname@example.org>, "Rafael J. Wysocki" <email@example.com>, firstname.lastname@example.org Subject: Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED_DEADLINE Date: Mon, 24 May 2021 21:47:06 +0100 [thread overview] Message-ID: <20210524204706.GE15545@willie-the-truck> (raw) In-Reply-To: <email@example.com> On Fri, May 21, 2021 at 03:00:42PM +0200, Daniel Bristot de Oliveira wrote: > 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). That's a really interesting point that I hadn't previously considered. It means that the effects of 32-bit tasks with forced affinity are far reaching when it comes to deadline 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. I've implemented AC on execve() of a 32-bit program so we'll fail that system call with -ENOEXEC if the root domain contains 64-bit-only CPUs. As expected, the failure mode is awful because it seems as though the ELF binary is then treated like a shell script by userspace and passed to /bin/sh: $ sudo chrt -d -T 5000000 -P 16666666 0 ./hello32 ./hello32: 1: Syntax error: word unexpected (expecting ")") Anyway, I'll include this in v7. Cheers, Will _______________________________________________ linux-arm-kernel mailing list firstname.lastname@example.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2021-05-25 2:09 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 2021-05-21 13:12 ` Quentin Perret 2021-05-24 20:47 ` Will Deacon [this message] 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 \ --in-reply-to=20210524204706.GE15545@willie-the-truck \ --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).