Re: [RFC][PATCH] another swap controller for cgroup

[Balbir Singh <balbir@linux.vnet.ibm.com>]

YAMAMOTO Takashi wrote:
> hi,
> 
>> Hi, Thanks for the patches and your patience. I've just applied your
>> patches on top of 2.6.25-mm1 (it had a minor reject, that I've fixed).
>> I am building and testing the patches along with KAMEZAWA-San's low
>> overhead patches.
> 
> thanks.
> 
>>> +#include <linux/err.h>
>>> +#include <linux/cgroup.h>
>>> +#include <linux/hugetlb.h>
>> My powerpc build fails, we need to move hugetlb.h down to the bottom
> 
> what's the error message?
> 

It's unable to find the hugetlb call, I think is_hugetlb_vma() or so.

>>> +struct swap_cgroup {
>>> +	struct cgroup_subsys_state scg_css;
>> Can't we call this just css. Since the structure is swap_cgroup it
>> already has the required namespace required to distinguish it from
>> other css's. Please see page 4 of "The practice of programming", be
>> consistent. The same comment applies to all members below.
> 
> i don't have the book.
> i like this kind of prefixes as it's grep-friendly.
> 
>>> +#define	task_to_css(task) task_subsys_state((task), swap_cgroup_subsys_id)
>>> +#define	css_to_scg(css)	container_of((css), struct swap_cgroup, scg_css)
>>> +#define	cg_to_css(cg)	cgroup_subsys_state((cg), swap_cgroup_subsys_id)
>>> +#define	cg_to_scg(cg)	css_to_scg(cg_to_css(cg))
>> Aren't static inline better than macros? I would suggest moving to
>> them.
> 
> sounds like a matter of preference.
> either ok for me.
> 

There are other advantages, like better type checking of the arguments. The
compiler might even determine that it's better of making a function call instead
of inlining it (rare, but possible).

>>> +static struct swap_cgroup *
>>> +swap_cgroup_prepare_ptp(struct page *ptp, struct mm_struct *mm)
>>> +{
>>> +	struct swap_cgroup *scg = ptp->ptp_swap_cgroup;
>>> +
>> Is this routine safe w.r.t. concurrent operations, modifications to
>> ptp_swap_cgroup?
> 
> it's always accessed with the page table locked.
> 
>>> +	BUG_ON(mm == NULL);
>>> +	BUG_ON(mm->swap_cgroup == NULL);
>>> +	if (scg == NULL) {
>>> +		/*
>>> +		 * see swap_cgroup_attach.
>>> +		 */
>>> +		smp_rmb();
>>> +		scg = mm->swap_cgroup;
>> With the mm->owner patches, we need not maintain a separate
>> mm->swap_cgroup.
> 
> i don't think the mm->owner patch, at least with the current form,
> can replace it.
> 

Could you please mention what the limitations are? We could get those fixed or
take another serious look at the mm->owner patches.

>>> +	/*
>>> +	 * swap_cgroup_attach is in progress.
>>> +	 */
>>> +
>>> +	res_counter_charge_force(&newscg->scg_counter, PAGE_CACHE_SIZE);
>> So, we force the counter to go over limit?
> 
> yes.
> 
> as newscg != oldscg here means the task is being moved between cgroups,
> this instance of res_counter_charge_force should not matter much.
> 

Isn't it bad to force a group to go over it's limit due to migration?

>>> +static int
>>> +swap_cgroup_write_u64(struct cgroup *cg, struct cftype *cft, u64 val)
>>> +{
>>> +	struct res_counter *counter = &cg_to_scg(cg)->scg_counter;
>>> +	unsigned long flags;
>>> +
>>> +	/* XXX res_counter_write_u64 */
>>> +	BUG_ON(cft->private != RES_LIMIT);
>>> +	spin_lock_irqsave(&counter->lock, flags);
>>> +	counter->limit = val;
>>> +	spin_unlock_irqrestore(&counter->lock, flags);
>>> +	return 0;
>>> +}
>>> +
>> We need to write actual numbers here? Can't we keep the write
>> interface consistent with the memory controller?
> 
> i'm not sure what you mean here.  can you explain a bit more?
> do you mean K, M, etc?
> 

Yes, I mean the same format that memparse() uses.

>>> +static void
>>> +swap_cgroup_destroy(struct cgroup_subsys *ss, struct cgroup *cg)
>>> +{
>>> +	struct swap_cgroup *oldscg = cg_to_scg(cg);
>>> +	struct swap_cgroup *newscg;
>>> +	struct list_head *pos;
>>> +	struct list_head *next;
>>> +
>>> +	/*
>>> +	 * move our anonymous objects to init_mm's group.
>>> +	 */
>> Is this good design, should be allow a swap_cgroup to be destroyed,
>> even though pages are allocated to it?
> 
> first of all, i'm not quite happy with this design. :)
> having said that, what else can we do?
> i tend to think that trying to swap-in these pages is too much effort
> for little benefit.
> 

Just fail the destroy operation, in this case.

>> Is moving to init_mm (root
>> cgroup) a good idea? Ideally with support for hierarchies, if we ever
>> do move things, it should be to the parent cgroup.
> 
> i chose init_mm because there seemed to be no consensus about
> cgroup hierarchy semantics.
> 

I would suggest that we fail deletion of a group for now. I have a set of
patches for the cgroup hierarchy semantics. I think the parent is the best place
to move it.

>>> +		info->swap_cgroup = newscg;
>>> +		res_counter_uncharge(&oldscg->scg_counter, bytes);
>>> +		res_counter_charge_force(&newscg->scg_counter, bytes);
>> I don't like the excessive use of res_counter_charge_force(), it seems
>> like we might end up bypassing the controller all together. I would
>> rather fail the destroy operation if the charge fails.
> 
>>> +	bytes = swslots * PAGE_CACHE_SIZE;
>>> +	res_counter_uncharge(&oldscg->scg_counter, bytes);
>>> +	/*
>>> +	 * XXX ignore newscg's limit because cgroup ->attach method can't fail.
>>> +	 */
>>> +	res_counter_charge_force(&newscg->scg_counter, bytes);
>> But in the future, we could plan on making attach fail (I have a
>> requirement for it). Again, I don't like the _force operation
> 
> allowing these operations fail implies to have code to back out
> partial operations.  i'm afraid that it will be too complex.
> 

OK, we need to find out a way to fix that then.

>>> +static void
>>> +swap_cgroup_attach_mm(struct mm_struct *mm, struct swap_cgroup *oldscg,
>>> +    struct swap_cgroup *newscg)
>> We need comments about the function, why do we attach an mm?
> 
> instead of a task, you mean?
> because we count the number of ptes which points to swap
> and ptes belong to an mm, not a task.
> 

OK

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

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