archive mirror
 help / color / mirror / Atom feed
From: Peter Zijlstra <>
To: Will Deacon <>
	Catalin Marinas <>,
	Marc Zyngier <>,
	Greg Kroah-Hartman <>,
	Morten Rasmussen <>,
	Qais Yousef <>,
	Suren Baghdasaryan <>,
	Quentin Perret <>, Tejun Heo <>,
	Li Zefan <>,
	Johannes Weiner <>,
	Ingo Molnar <>,
	Juri Lelli <>,
	Vincent Guittot <>,
Subject: Re: [PATCH v5 00/15] An alternative series for asymmetric AArch32 systems
Date: Tue, 15 Dec 2020 18:36:45 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, Dec 08, 2020 at 01:28:20PM +0000, Will Deacon wrote:
> The aim of this series is to allow 32-bit ARM applications to run on
> arm64 SoCs where not all of the CPUs support the 32-bit instruction set.
> Unfortunately, such SoCs are real and will continue to be productised
> over the next few years at least. I can assure you that I'm not just
> doing this for fun.
> Changes in v5 include:
>   * Teach cpuset_cpus_allowed() about task_cpu_possible_mask() so that
>     we can avoid returning incompatible CPUs for a given task. This
>     means that sched_setaffinity() can be used with larger masks (like
>     the online mask) from userspace and also allows us to take into
>     account the cpuset hierarchy when forcefully overriding the affinity
>     for a task on execve().
>   * Honour task_cpu_possible_mask() when attaching a task to a cpuset,
>     so that the resulting affinity mask does not contain any incompatible
>     CPUs (since it would be rejected by set_cpus_allowed_ptr() otherwise).
>   * Moved overriding of the affinity mask into the scheduler core rather
>     than munge affinity masks directly in the architecture backend.

Hurmph... so if I can still read, this thing will auto truncate the
affinity mask to something that only contains compatible CPUs, right?

Assuming our system has 8 CPUs (0xFF), half of which are 32bit capable
(0x0F), then, when our native task (with affinity 0x3c) does a
fork()+execve() of a 32bit thingy the resulting task has 0x0c.

If that in turn does fork()+execve() of a native task, it will retain
the trucated affinity mask (0x0c), instead of returning to the wider
mask (0x3c).

IOW, any (accidental or otherwise) trip through a 32bit helper, will
destroy user state (the affinity mask: 0x3c).

Should we perhaps split task_struct::cpus_mask, one to keep an original
copy of the user state, and one to be an effective cpumask for the task?
That way, the moment a task constricts or widens it's
task_cpu_possible_mask() we can re-compute the effective mask without
loss of information.

  parent reply	other threads:[~2020-12-15 17:38 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-08 13:28 [PATCH v5 00/15] An alternative series for asymmetric AArch32 systems Will Deacon
2020-12-08 13:28 ` [PATCH v5 01/15] arm64: cpuinfo: Split AArch32 registers out into a separate struct Will Deacon
2020-12-08 13:28 ` [PATCH v5 02/15] arm64: Allow mismatched 32-bit EL0 support Will Deacon
2020-12-08 13:28 ` [PATCH v5 03/15] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched " Will Deacon
2020-12-08 13:28 ` [PATCH v5 04/15] arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs Will Deacon
2020-12-08 13:28 ` [PATCH v5 05/15] arm64: Advertise CPUs capable of running 32-bit applications in sysfs Will Deacon
2020-12-08 13:28 ` [PATCH v5 06/15] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection Will Deacon
2020-12-08 13:28 ` [PATCH v5 07/15] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1 Will Deacon
2020-12-17 12:15   ` Qais Yousef
2020-12-17 13:44     ` Peter Zijlstra
2020-12-17 14:59       ` Will Deacon
2020-12-17 15:00       ` Qais Yousef
2020-12-08 13:28 ` [PATCH v5 08/15] cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus() Will Deacon
2020-12-28  3:54   ` Suren Baghdasaryan
2020-12-08 13:28 ` [PATCH v5 09/15] sched: Reject CPU affinity changes based on task_cpu_possible_mask() Will Deacon
2020-12-08 13:28 ` [PATCH v5 10/15] sched: Introduce force_compatible_cpus_allowed_ptr() to limit CPU affinity Will Deacon
2020-12-28  4:29   ` Suren Baghdasaryan
2020-12-08 13:28 ` [PATCH v5 11/15] arm64: Implement task_cpu_possible_mask() Will Deacon
2020-12-08 13:28 ` [PATCH v5 12/15] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0 Will Deacon
2020-12-08 13:28 ` [PATCH v5 13/15] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system Will Deacon
2020-12-08 13:28 ` [PATCH v5 14/15] arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0 Will Deacon
2020-12-08 13:28 ` [PATCH v5 15/15] arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores Will Deacon
2020-12-15 17:36 ` Peter Zijlstra [this message]
2020-12-15 18:50   ` [PATCH v5 00/15] An alternative series for asymmetric AArch32 systems Will Deacon
2020-12-17 10:55     ` Peter Zijlstra
2020-12-16 11:16 ` Qais Yousef
2020-12-16 14:14   ` Will Deacon
2020-12-16 16:48     ` Qais Yousef
2020-12-16 18:21       ` Suren Baghdasaryan

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:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
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).