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Date: Sun, 3 Jan 2016 08:59:11 -0500
From: Tejun Heo <tj@kernel.org>
To: Chen Yu <yu.c.chen@intel.com>
Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        Vlastimil Babka <vbabka@suse.cz>, Rik van Riel <riel@redhat.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        David Rientjes <rientjes@google.com>,
        Vishnu Pratap Singh <vishnu.ps@samsung.com>,
        Pintu Kumar <pintu.k@samsung.com>,
        Michal Nazarewicz <mina86@mina86.com>, Mel Gorman <mgorman@suse.de>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>, Hugh Dickins <hughd@google.com>,
        Li Zefan <lizefan@huawei.com>
Subject: Re: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs
Message-ID: <20160103135911.GG3660@htj.duckdns.org>
References: <2f805346f87680dddfa9253dc9031db3b6ead8ad.1451648318.git.yu.c.chen@intel.com>
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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