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

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Message-ID: <20160103135911.GG3660@htj.duckdns.org>
Date:	Sun, 3 Jan 2016 08:59:11 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Chen Yu <yu.c.chen@...el.com>
Cc:	cgroups@...r.kernel.org, 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>, Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

On Fri, Jan 01, 2016 at 08:09:13PM +0800, 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 patch works around this problem by introducing a new
> mask cpumask_var_t cpus_sysfs inside struct cpuset,
> which will only be updated by writing value to sysfs.cpuset.cpus,
> and CPU offline/online will use this mask to set the new cpumask
> for a cpuset.

Li?

-- 
tejun
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