From: Qais Yousef <firstname.lastname@example.org>
To: Will Deacon <email@example.com>
Cc: 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>,
Quentin Perret <email@example.com>, Tejun Heo <firstname.lastname@example.org>,
Li Zefan <email@example.com>,
Johannes Weiner <firstname.lastname@example.org>,
Ingo Molnar <email@example.com>,
Juri Lelli <firstname.lastname@example.org>,
Vincent Guittot <email@example.com>,
Subject: Re: [PATCH v5 00/15] An alternative series for asymmetric AArch32 systems
Date: Wed, 16 Dec 2020 16:48:45 +0000 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
On 12/16/20 14:14, Will Deacon wrote:
> Hi Qais,
> On Wed, Dec 16, 2020 at 11:16:46AM +0000, Qais Yousef wrote:
> > On 12/08/20 13:28, Will Deacon wrote:
> > > 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.
> > >
> > > * Extended comments and documentation.
> > >
> > > * Some renaming and cosmetic changes.
> > >
> > > I'm pretty happy with this now, although it still needs review and will
> > > require rebasing to play nicely with the SCA changes in -next.
> > I still have concerns about the cpuset v1 handling. Specifically:
> > 1. Attaching a 32bit task to 64bit only cpuset is allowed.
> > I think the right behavior here is to prevent that as the
> > intersection will appear as offline cpus for the 32bit tasks. So it
> > shouldn't be allowed to move there.
> Suren or Quantin can correct me if I'm wrong I'm here, but I think Android
> relies on this working so it's not an option for us to prevent the attach.
I don't think so. It's just a matter who handles the error. ie: kernel fix it
up silently and effectively make the cpuset a NOP since we don't respect the
affinity of the cpuset, or user space pick the next best thing. Since this
could return an error anyway, likely user space already handles this.
> I also don't think it really achieves much, since as you point out, the same
> problem exists in other cases such as execve() of a 32-bit binary, or
> hotplugging off all 32-bit CPUs within a mixed cpuset. Allowing the attach
> and immediately reparenting would probably be better, but see below.
I am just wary that we're introducing a generic asymmetric ISA support, so my
concerns have been related to making sure the behavior is sane generally. When
this gets merged, I can bet more 'fun' hardware will appear all over the place.
We're opening the flood gates I'm afraid :p
> > 2. Modifying cpuset.cpus could result with empty set for 32bit tasks.
> > It is a variation of the above, it's just the cpuset transforms into
> > 64bit only after we attach.
> > I think the right behavior here is to move the 32bit tasks to the
> > nearest ancestor like we do when all cpuset.cpus are hotplugged out.
> > We could too return an error if the new set will result an empty set
> > for the 32bit tasks. In a similar manner to how it fails if you
> > write a cpu that is offline.
> > 3. If a 64bit task belongs to 64bit-only-cpuset execs a 32bit binary,
> > the 32 tasks will inherit the cgroup setting.
> > Like above, we should move this to the nearest ancestor.
> I considered this when I was writing the patches, but the reality is that
> by allowing 32-bit tasks to attach to a 64-bit only cpuset (which is required
> by Android), we have no choice but to expose a new ABI to userspace. This is
> all gated behind a command-line option, so I think that's fine, but then why
> not just have the same behaviour as cgroup v2? I don't see the point in
> creating two new ABIs (for cgroup v1 and v2 respectively) if we don't need
Ultimately it's up to Tejun and Peter I guess. I thought we need to preserve
the v1 behavior for the new class of tasks. I won't object to the new ABI
myself. Maybe we just need to make the commit messages and cgroup-v1
documentation reflect that explicitly.
> to. If it was _identical_ to the hotplug case, then we would surely just
> follow the existing behaviour, but it's really quite different in this
> situation because the cpuset is not empty.
It is actually effectively empty for those tasks. But I see that one could look
at it from two different angles.
> One thing we should definitely do though is add this to the documentation
> for the command-line option.
By the way, should the command-line option be renamed to something more
generic? This has already grown beyond just enabling the support for one
isolated case. No strong opinion, just a suggestion.
next prev parent reply other threads:[~2020-12-16 16:49 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 ` [PATCH v5 00/15] An alternative series for asymmetric AArch32 systems Peter Zijlstra
2020-12-15 18:50 ` 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 [this message]
2020-12-16 18:21 ` Suren Baghdasaryan
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).