Re: [RFC PATCH 5/8] hugetlb: change free_pool_huge_page to remove_pool_huge_page

[Mike Kravetz <mike.kravetz@oracle.com>]

On 3/23/21 12:57 AM, Michal Hocko wrote:
> On Mon 22-03-21 16:28:07, Mike Kravetz wrote:
>> On 3/22/21 7:31 AM, Michal Hocko wrote:
>>> On Fri 19-03-21 15:42:06, Mike Kravetz wrote:
>>> [...]
>>>> @@ -2090,9 +2084,15 @@ static void return_unused_surplus_pages(struct hstate *h,
>>>>  	while (nr_pages--) {
>>>>  		h->resv_huge_pages--;
>>>>  		unused_resv_pages--;
>>>> -		if (!free_pool_huge_page(h, &node_states[N_MEMORY], 1))
>>>> +		page = remove_pool_huge_page(h, &node_states[N_MEMORY], 1);
>>>> +		if (!page)
>>>>  			goto out;
>>>> -		cond_resched_lock(&hugetlb_lock);
>>>> +
>>>> +		/* Drop lock and free page to buddy as it could sleep */
>>>> +		spin_unlock(&hugetlb_lock);
>>>> +		update_and_free_page(h, page);
>>>> +		cond_resched();
>>>> +		spin_lock(&hugetlb_lock);
>>>>  	}
>>>>  
>>>>  out:
>>>
>>> This is likely a matter of taste but the repeated pattern of unlock,
>>> update_and_free_page, cond_resched and lock seems rather clumsy.
>>> Would it be slightly better/nicer to remove_pool_huge_page into a
>>> list_head under a single lock invocation and then free up the whole lot
>>> after the lock is dropped?
>>
>> Yes, we can certainly do that.
>> One downside I see is that the list can contain a bunch of pages not
>> accounted for in hugetlb and not free in buddy (or cma).  Ideally, we
>> would want to keep those in sync if possible.  Also, the commit that
>> added the cond_resched talked about freeing up 12 TB worth of huge pages
>> and it holding the lock for 150 seconds.  The new code is not holding
>> the lock while calling free to buddy, but I wonder how long it would
>> take to remove 12 TB worth of huge pages and add them to a separate list?
> 
> Well, the remove_pool_huge_page is just a accounting part and that
> should be pretty invisible even when the number of pages is large. The
> lockless nature (from hugetlb POV) of the final page release is the
> heavy weight operation and whether you do it in chunks or in a single go
> (with cond_resched) should be visible either. We already do the same
> thing when uncharging memcg pages (mem_cgroup_uncharge_list). 
> 
> So I would agree with you that this would be a much bigger problem if
> both the hugetlb and freeing path were equally heavy weight and the
> delay between first pages uncaccounted and freed would be noticeable.
> 
> But I do not want to push for this. I just hated the hugetlb_lock dances
> as this is ugly and repetitive pattern.

As you may have seen in my reply to patch 3, I am going to use this
batching approach for all places we do remove/free hugetlb page.

Since you brought up cgroups ... what is your opinion on lock hold time
in hugetlb_cgroup_css_offline?  We could potentially be calling
hugetlb_cgroup_move_parent for every hugetlb page while holding the lock
with interrupts disabled.
-- 
Mike Kravetz