From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org, kernel-team@android.com,
Zefan Li <lizefan.x@bytedance.com>, Tejun Heo <tj@kernel.org>,
Johannes Weiner <hannes@cmpxchg.org>,
cgroups@vger.kernel.org
Subject: Re: [PATCH 1/2] cpuset: Fix cpuset_cpus_allowed() to not filter offline CPUs
Date: Wed, 1 Feb 2023 22:10:46 +0100 [thread overview]
Message-ID: <Y9rVVldS19oyIZ+g@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <a892d340-ea99-1562-0e70-176f02f195c2@redhat.com>
On Wed, Feb 01, 2023 at 01:46:11PM -0500, Waiman Long wrote:
> Note that using cpus_allowed directly in cgroup v2 may not be right because
> cpus_allowed may have no relationship to effective_cpus at all in some
> cases, e.g.
>
> root
> |
> V
> A (cpus_allowed = 1-4, effective_cpus = 1-4)
> |
> V
> B (cpus_allowed = 5-8, effective_cpus = 1-4)
>
> In the case of cpuset B, passing back cpus 5-8 as the allowed_cpus is wrong.
I think my patch as written does the right thing here. Since the
intersection of (1-4) and (5-8) is empty it will move up the hierarchy
and we'll end up with (1-4) from the cgroup side of things.
So the purpose of __cs_cpus_allowed() is to override the cpus_allowed of
the root set and force it to cpu_possible_mask.
Then cs_cpus_allowed() computes the intersection of cs->cpus_allowed and
all it's parents. This will, in the case of B above, result in the empty
mask.
Then cpuset_cpus_allowed() has a loop that starts with
task_cpu_possible_mask(), intersects that with cs_cpus_allowed() and if
the intersection of that and cpu_online_mask is empty, moves up the
hierarchy. Given cs_cpus_allowed(B) is the empty mask, we'll move to A.
Note that since we force the mask of root to cpu_possible_mask,
cs_cpus_allowed(root) will be a no-op and if we guarantee (in arch code)
that cpu_online_mask always has a non-empty intersection with
task_cpu_possible_mask(), this loop is guaranteed to terminate with a
viable mask.
next prev parent reply other threads:[~2023-02-01 21:11 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-31 22:17 [PATCH 0/2] Fix broken cpuset affinity handling on heterogeneous systems Will Deacon
2023-01-31 22:17 ` [PATCH 1/2] cpuset: Fix cpuset_cpus_allowed() to not filter offline CPUs Will Deacon
2023-02-01 4:14 ` Waiman Long
2023-02-01 9:14 ` Peter Zijlstra
2023-02-01 15:16 ` Waiman Long
2023-02-01 18:46 ` Waiman Long
2023-02-01 19:14 ` Waiman Long
2023-02-01 19:17 ` Waiman Long
2023-02-01 21:10 ` Peter Zijlstra [this message]
2023-02-02 3:34 ` Waiman Long
2023-02-03 11:50 ` Will Deacon
2023-02-03 15:13 ` Waiman Long
2023-02-03 15:26 ` Peter Zijlstra
2023-02-03 15:35 ` Waiman Long
2023-02-02 8:34 ` Peter Zijlstra
2023-02-02 16:06 ` Waiman Long
2023-02-02 19:42 ` Peter Zijlstra
2023-02-02 20:46 ` Waiman Long
2023-02-02 20:48 ` Tejun Heo
2023-02-02 20:53 ` Waiman Long
2023-02-02 21:05 ` Waiman Long
2023-02-02 21:50 ` Tejun Heo
2023-02-03 0:54 ` Waiman Long
2023-02-03 16:31 ` Will Deacon
2023-01-31 22:17 ` [PATCH 2/2] cpuset: Call set_cpus_allowed_ptr() with appropriate mask for task Will Deacon
2023-02-01 2:22 ` Waiman Long
2023-02-01 9:15 ` Peter Zijlstra
2023-02-01 15:03 ` Waiman Long
2023-02-01 9:27 ` Peter Zijlstra
2023-02-03 17:55 ` Waiman Long
2023-02-06 20:21 ` Tejun Heo
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