linux-kernel - Re: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

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Date:	Mon, 4 Jan 2016 09:52:09 +0800
From:	Zefan Li <lizefan@...wei.com>
To:	Chen Yu <yu.c.chen@...el.com>, <cgroups@...r.kernel.org>
CC:	<linux-kernel@...r.kernel.org>, Vlastimil Babka <vbabka@...e.cz>,
	"Rik van Riel" <riel@...hat.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	David Rientjes <rientjes@...gle.com>,
	Vishnu Pratap Singh <vishnu.ps@...sung.com>,
	"Pintu Kumar" <pintu.k@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	"Mel Gorman" <mgorman@...e.de>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

On 2016/1/1 20:09, Chen Yu wrote:
> Commit be4c9dd7aee5 ("cpuset: enable onlined cpu/node in effective masks")
> leverages cpuset's cpus_allowed and its parent's effective_cpus to calculate
> the new_cpus by:
>
> cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);
>
> However cpus_allowed will also be updated after the CPU is offline, in
> hotplug_update_tasks_legacy, so when the CPU is online again, it will use
> the old cpus_allowed mask to calculate the new_cpus, thus new_cpus will get
> incorrect value after each round of offline/online.
>
> This problem is found on ubuntu 15.10 with cpuset mounted:
>
> 1. echo 0 > /sys/devices/system/cpu/cpu2/online
> 2. echo 1 > /sys/devices/system/cpu/cpu2/online
> 3. cat /sys/fs/cgroup/cpuset/cpuset.cpus
>     0-3
> 4. cat /sys/fs/cgroup/cpuset/user.slice/cpuset.cpus
>     0-1,3
> 5. taskset -c 2 ls
>     taskset: failed to set pid 0's affinity: Invalid argument
>

This is the expected behavior...In legacy hierachy onlining an offlined cpu
won't restore cpuset configurations automatically.

Commit be4c9dd7aee5 changed this behavior, but only for unified hierachy.

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