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From: Will Deacon <will@kernel.org>
To: Juri Lelli <juri.lelli@redhat.com>
Cc: Quentin Perret <qperret@google.com>,
	linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Qais Yousef <qais.yousef@arm.com>,
	Suren Baghdasaryan <surenb@google.com>, Tejun Heo <tj@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Ingo Molnar <mingo@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	kernel-team@android.com,
	Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED_DEADLINE
Date: Thu, 20 May 2021 11:16:41 +0100	[thread overview]
Message-ID: <20210520101640.GA10065@willie-the-truck> (raw)
In-Reply-To: <YKYoQ0ezahSC/RAg@localhost.localdomain>

Hi Juri,

On Thu, May 20, 2021 at 11:13:39AM +0200, Juri Lelli wrote:
> Apologies for the delay in replying.

Not at all!

> On 18/05/21 13:19, Quentin Perret wrote:
> > On Tuesday 18 May 2021 at 11:59:51 (+0100), Will Deacon wrote:
> > > On Tue, May 18, 2021 at 10:48:07AM +0000, Quentin Perret wrote:
> > > > On Tuesday 18 May 2021 at 11:28:34 (+0100), Will Deacon wrote:
> > > > > I don't have strong opinions on this, but I _do_ want the admission via
> > > > > sched_setattr() to be consistent with execve(). What you're suggesting
> > > > > ticks that box, but how many applications are prepared to handle a failed
> > > > > execve()? I suspect it will be fatal.
> > > > 
> > > > Yep, probably.
> > > > 
> > > > > Probably also worth pointing out that the approach here will at least
> > > > > warn in the execve() case when the affinity is overridden for a deadline
> > > > > task.
> > > > 
> > > > Right so I think either way will be imperfect, so I agree with the
> > > > above.
> > > > 
> > > > Maybe one thing though is that, IIRC, userspace _can_ disable admission
> > > > control if it wants to. In this case I'd have no problem with allowing
> > > > this weird behaviour when admission control is off -- the kernel won't
> > > > provide any guarantees. But if it's left on, then it's a different
> > > > story.
> > > > 
> > > > So what about we say, if admission control is off, we allow execve() and
> > > > sched_setattr() with appropriate warnings as you suggest, but if
> > > > admission control is on then we fail both?
> > > 
> > > That's an interesting idea. The part that I'm not super keen about is
> > > that it means admission control _also_ has an effect on the behaviour of
> > > execve()
> > 
> > Right, that's a good point. And it looks like fork() behaves the same
> > regardless of admission control being enabled or not -- it is forbidden
> > from DL either way. So I can't say there is a precedent :/
> > 
> > > so practically you'd have to have it disabled as long as you
> > > have the possibility of 32-bit deadline tasks anywhere in the system,
> > > which impacts 64-bit tasks which may well want admission control enabled.
> > 
> > Indeed, this is a bit sad, but I don't know if the kernel should pretend
> > it can guarantee to meet your deadlines and at the same time allow to do
> > something that wrecks the underlying theory.
> > 
> > I'd personally be happy with saying that admission control should be
> > disabled on these dumb systems (and have that documented), at least
> > until DL gets proper support for affinities. ISTR there was work going
> > in that direction, but some folks in the CC list will know better.
> > 
> > @Juri, maybe you would know if that's still planned?
> 
> I won't go as far as saying planned, but that is still under "our" radar
> for sure. Daniel was working on it, but I don't think he had any time to
> resume that bit of work lately.
> 
> So, until we have that, I think we have been as conservative as we could
> for this type of decisions. I'm a little afraid that allowing
> configuration to break admission control (even with a non fatal warning
> is emitted) is still risky. I'd go with fail hard if AC is on, let it
> pass if AC is off (supposedly the user knows what to do). But I'm not
> familiar with the mixed 32/64 apps usecase you describe, so I might be
> missing details.

Ok, thanks for the insight. In which case, I'll go with what we discussed:
require admission control to be disabled for sched_setattr() but allow
execve() to a 32-bit task from a 64-bit deadline task with a warning (this
is probably similar to CPU hotplug?).

I'll update that for v8, and this patch will disappear.

Will

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  reply	other threads:[~2021-05-20 10:18 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 [this message]
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
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

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