linux-kernel - Re: [PATCH 3/6] cpuset: convert cpuset_attach() to use cpumask_var

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Date:	Mon, 05 Jan 2009 17:04:54 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...e.hu>, Rusty Russell <rusty@...tcorp.com.au>,
	Mike Travis <travis@....com>, Paul Menage <menage@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/6] cpuset: convert cpuset_attach() to use cpumask_var_t

Andrew Morton wrote:
> On Mon, 05 Jan 2009 16:47:21 +0800 Li Zefan <lizf@...fujitsu.com> wrote:
> 
>> Allocate a global cpumask_var_t at boot, and use it in cpuset_attach(), so
>> we won't fail cpuset_attach().
>>
>> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
>> Acked-by: Mike Travis <travis@....com>
>> ---
>>  kernel/cpuset.c |   14 +++++++++-----
>>  1 files changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
>> index afa29cf..1e32e6b 100644
>> --- a/kernel/cpuset.c
>> +++ b/kernel/cpuset.c
>> @@ -1306,6 +1306,9 @@ static int fmeter_getrate(struct fmeter *fmp)
>>  	return val;
>>  }
>>  
>> +/* Protected by cgroup_lock */
>> +static cpumask_var_t cpus_attach;
>> +
>>  /* Called by cgroups to determine if a cpuset is usable; cgroup_mutex held */
>>  static int cpuset_can_attach(struct cgroup_subsys *ss,
>>  			     struct cgroup *cont, struct task_struct *tsk)
>> @@ -1330,7 +1333,6 @@ static void cpuset_attach(struct cgroup_subsys *ss,
>>  			  struct cgroup *cont, struct cgroup *oldcont,
>>  			  struct task_struct *tsk)
>>  {
>> -	cpumask_t cpus;
>>  	nodemask_t from, to;
>>  	struct mm_struct *mm;
>>  	struct cpuset *cs = cgroup_cs(cont);
>> @@ -1338,13 +1340,13 @@ static void cpuset_attach(struct cgroup_subsys *ss,
>>  	int err;
>>  
>>  	if (cs == &top_cpuset) {
>> -		cpus = cpu_possible_map;
>> +		cpumask_copy(cpus_attach, cpu_possible_mask);
>>  	} else {
>>  		mutex_lock(&callback_mutex);
>> -		guarantee_online_cpus(cs, &cpus);
>> +		guarantee_online_cpus(cs, cpus_attach);
>>  		mutex_unlock(&callback_mutex);
>>  	}
>> -	err = set_cpus_allowed_ptr(tsk, &cpus);
>> +	err = set_cpus_allowed_ptr(tsk, cpus_attach);
>>  	if (err)
>>  		return;
>>  
>> @@ -1357,7 +1359,6 @@ static void cpuset_attach(struct cgroup_subsys *ss,
>>  			cpuset_migrate_mm(mm, &from, &to);
>>  		mmput(mm);
>>  	}
>> -
>>  }
>>  
>>  /* The various types of files and directories in a cpuset file system */
>> @@ -1838,6 +1839,9 @@ int __init cpuset_init(void)
>>  	if (err < 0)
>>  		return err;
>>  
>> +	if (!alloc_cpumask_var(&cpus_attach, GFP_KERNEL))
>> +		BUG();
>> +
>>  	number_of_cpusets = 1;
>>  	return 0;
>>  }
> 
> OK, that works.
> 
> Do we need to dynamically allocate cpus_attach?  Can we just do
> 
> static cpumask_t cpus_attach;
> 
> ?
> 

Yes, it's used by cpuset_attach() only, and cpuset_attach() is called with
cgroup_lock() held, so it won't happen that 2 threads access cpus_attach
concurrently.

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