Re: [-mm] Add an owner to the mm_struct (v7)

[Balbir Singh]

Dave Hansen wrote:
> On Thu, 2008-04-03 at 23:14 +0530, Balbir Singh wrote:
>> diff -puN include/linux/init_task.h~memory-controller-add-mm-owner include/linux/init_task.h
>> --- linux-2.6.25-rc8/include/linux/init_task.h~memory-controller-add-mm-owner	2008-04-03 22:43:27.000000000 +0530
>> +++ linux-2.6.25-rc8-balbir/include/linux/init_task.h	2008-04-03 22:43:27.000000000 +0530
>> @@ -199,7 +199,6 @@ extern struct group_info init_groups;
>>  	INIT_LOCKDEP							\
>>  }
>>
>> -
>>  #define INIT_CPU_TIMERS(cpu_timers)					\
>>  {									\
>>  	LIST_HEAD_INIT(cpu_timers[0]),					\
> 
> I assume you didn't mean to do that one.
> 

No, I made some changes to that file and the undo removed an extra space, which
showed up. Thanks for spotting it.

>> diff -puN include/linux/memcontrol.h~memory-controller-add-mm-owner include/linux/memcontrol.h
>> --- linux-2.6.25-rc8/include/linux/memcontrol.h~memory-controller-add-mm-owner	2008-04-03 22:43:27.000000000 +0530
>> +++ linux-2.6.25-rc8-balbir/include/linux/memcontrol.h	2008-04-03 22:43:27.000000000 +0530
>> @@ -27,9 +27,6 @@ struct mm_struct;
>>
>>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR
>>
>> -extern void mm_init_cgroup(struct mm_struct *mm, struct task_struct *p);
>> -extern void mm_free_cgroup(struct mm_struct *mm);
>> -
>>  #define page_reset_bad_cgroup(page)	((page)->page_cgroup = 0)
>>
>>  extern struct page_cgroup *page_get_page_cgroup(struct page *page);
>> @@ -48,8 +45,10 @@ extern unsigned long mem_cgroup_isolate_
>>  extern void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask);
>>  int task_in_mem_cgroup(struct task_struct *task, const struct mem_cgroup *mem);
>>
>> +extern struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);
>> +
>>  #define mm_match_cgroup(mm, cgroup)	\
>> -	((cgroup) == rcu_dereference((mm)->mem_cgroup))
>> +	((cgroup) == mem_cgroup_from_task((mm)->owner))
> 
> Now that you've mucked with this one, can you just turn this into a
> static inline?
> 

It was a static inline, but there was a build error on sparc. David Reintjes
made this change to make it compile. This might not be the right patch for the
change you are suggesting, but definitely worth doing.

> ...
>> +#ifdef CONFIG_MM_OWNER
>> +/*
>> + * Task p is exiting and it owned p, so lets find a new owner for it
>> + */
>> +static inline int
>> +mm_need_new_owner(struct mm_struct *mm, struct task_struct *p)
>> +{
>> +	int ret;
>> +
>> +	/*
>> +	 * If there are other users of the mm and the owner (us) is exiting
>> +	 * we need to find a new owner to take on the responsibility.
>> +	 * When we use thread groups (CLONE_THREAD), the thread group
>> +	 * leader is kept around in zombie state, even after it exits.
>> +	 * delay_group_leader() ensures that if the group leader is around
>> +	 * we need not select a new owner.
>> +	 */
>> +	ret = (mm && (atomic_read(&mm->mm_users) > 1) && (mm->owner == p) &&
>> +		!delay_group_leader(p));
>> +	return ret;
>> +}
> 
> Ugh.  Could you please spell this out a bit more.  I find that stuff
> above really hard to read.  Something like:
> 
> 	if (!mm)
> 		return 0;
> 	if (atomic_read(&mm->mm_users) <= 1)
> 		return 0;
> 	if (mm->owner != p)
> 		return 0;
> 	if (delay_group_leader(p))
> 		return 0;
> 	return 1;
> 

The problem with code above is 4 branch instructions and the code I have just 4
AND operations. I don't think &&'s are so hard to read. If there is a mixture of
operations (&&, ||) then it can get a little harder


> It also gives you a nice spot to stick comments for each particular
> check.
> 
>> +void mm_update_next_owner(struct mm_struct *mm)
>> +{
>> +	struct task_struct *c, *g, *p = current;
> 
> Any chance I can talk you into spelling these out a bit?  By the time I
> get down in the function, it's easy to forget what they are.
> 

That can be done, will do

>> +retry:
>> +	if (!mm_need_new_owner(mm, p))
>> +		return;
>> +
>> +	rcu_read_lock();
>> +	/*
>> +	 * Search in the children
>> +	 */
>> +	list_for_each_entry(c, &p->children, sibling) {
>> +		if (c->mm == mm)
>> +			goto assign_new_owner;
>> +	}
>> +
>> +	/*
>> +	 * Search in the siblings
>> +	 */
>> +	list_for_each_entry(c, &p->parent->children, sibling) {
>> +		if (c->mm == mm)
>> +			goto assign_new_owner;
>> +	}
>> +
>> +	/*
>> +	 * Search through everything else. We should not get
>> +	 * here often
>> +	 */
>> +	do_each_thread(g, c) {
>> +		if (c->mm == mm)
>> +			goto assign_new_owner;
>> +	} while_each_thread(g, c);
> 
> What is the case in which we get here?  Threading that's two deep where
> none of the immeidate siblings or children is still alive?
> 

This usually happens for cases where threads were created without CLONE_THREAD.
We need to scan for shared mm's between processes (siblings and children scans
have not been successful).

> Have you happened to instrument this and see if it happens in practice
> much?
> 

Yes, I have. I removed the !delay_group_leader() and registered the cgroup
mm_owner_changed callback and saw the mm->owner change.

>> +	rcu_read_unlock();
>> +	return;
>> +
>> +assign_new_owner:
>> +	BUG_ON(c == p);
>> +	get_task_struct(c);
>> +	/*
>> +	 * The task_lock protects c->mm from changing.
>> +	 * We always want mm->owner->mm == mm
>> +	 */
>> +	task_lock(c);
>> +	/*
>> + 	 * Delay rcu_read_unlock() till we have the task_lock()
>> + 	 * to ensure that c does not slip away underneath us
>> + 	 */
>> +	rcu_read_unlock();
>> +	if (c->mm != mm) {
>> +		task_unlock(c);
>> +		put_task_struct(c);
>> +		goto retry;
>> +	}
>> +	cgroup_mm_owner_callbacks(mm->owner, c);
>> +	mm->owner = c;
>> +	task_unlock(c);
>> +	put_task_struct(c);
>> +}
>> +#endif /* CONFIG_MM_OWNER */

Thanks for the review,

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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