Re: [PATCH 3/6] cpuset: convert cpuset_attach() to use cpumask_var_t

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 05 Jan 2009 17:04:54 +0800 Li Zefan <lizf@cn.fujitsu.com> wrote:

> Andrew Morton wrote:
> > On Mon, 05 Jan 2009 16:47:21 +0800 Li Zefan <lizf@cn.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@cn.fujitsu.com>
> >> Acked-by: Mike Travis <travis@sgi.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.

You misunderstand my question.  I think.

Can we allocate cpus_attach at compile time?  Completely, not
partially.  By doing

static cpumask_t cpus_attach;

instead of

static cpumask_var_t cpus_attach;
...
	alloc_cpumask_var(&cpus_attach, GFP_KERNEL);

?