Re: [PATCH v2 1/2] THP: avoid lock when check whether THP is in deferred list

["Yin, Fengwei" <fengwei.yin@intel.com>]

Hi Kirill,

On 4/28/2023 10:02 PM, Kirill A. Shutemov wrote:
> On Fri, Apr 28, 2023 at 02:28:07PM +0800, Yin, Fengwei wrote:
>> Hi Kirill,
>>
>> On 4/25/2023 8:38 PM, Kirill A. Shutemov wrote:
>>> On Tue, Apr 25, 2023 at 04:46:26PM +0800, Yin Fengwei wrote:
>>>> free_transhuge_page() acquires split queue lock then check
>>>> whether the THP was added to deferred list or not.
>>>>
>>>> It's safe to check whether the THP is in deferred list or not.
>>>>    When code hit free_transhuge_page(), there is no one tries
>>>>    to update the folio's _deferred_list.
>>>>
>>>>    If folio is not in deferred_list, it's safe to check without
>>>>    acquiring lock.
>>>>
>>>>    If folio is in deferred_list, the other node in deferred_list
>>>>    adding/deleteing doesn't impact the return value of
>>>>    list_epmty(@folio->_deferred_list).
>>>
>>> Typo.
>>>
>>>>
>>>> Running page_fault1 of will-it-scale + order 2 folio for anonymous
>>>> mapping with 96 processes on an Ice Lake 48C/96T test box, we could
>>>> see the 61% split_queue_lock contention:
>>>> -   71.28%     0.35%  page_fault1_pro  [kernel.kallsyms]           [k]
>>>>     release_pages
>>>>    - 70.93% release_pages
>>>>       - 61.42% free_transhuge_page
>>>>          + 60.77% _raw_spin_lock_irqsave
>>>>
>>>> With this patch applied, the split_queue_lock contention is less
>>>> than 1%.
>>>>
>>>> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
>>>> Tested-by: Ryan Roberts <ryan.roberts@arm.com>
>>>> ---
>>>>  mm/huge_memory.c | 19 ++++++++++++++++---
>>>>  1 file changed, 16 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 032fb0ef9cd1..c620f1f12247 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -2799,12 +2799,25 @@ void free_transhuge_page(struct page *page)
>>>>  	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>>>>  	unsigned long flags;
>>>>  
>>>> -	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>>>> -	if (!list_empty(&folio->_deferred_list)) {
>>>> +	/*
>>>> +	 * At this point, there is no one trying to queue the folio
>>>> +	 * to deferred_list. folio->_deferred_list is not possible
>>>> +	 * being updated.
>>>> +	 *
>>>> +	 * If folio is already added to deferred_list, add/delete to/from
>>>> +	 * deferred_list will not impact list_empty(&folio->_deferred_list).
>>>> +	 * It's safe to check list_empty(&folio->_deferred_list) without
>>>> +	 * acquiring the lock.
>>>> +	 *
>>>> +	 * If folio is not in deferred_list, it's safe to check without
>>>> +	 * acquiring the lock.
>>>> +	 */
>>>> +	if (data_race(!list_empty(&folio->_deferred_list))) {
>>>> +		spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>>>
>>> Recheck under lock?
>> In function deferred_split_scan(), there is following code block:
>>                 if (folio_try_get(folio)) {
>>                         list_move(&folio->_deferred_list, &list);
>>                 } else {
>>                         /* We lost race with folio_put() */
>>                         list_del_init(&folio->_deferred_list);
>>                         ds_queue->split_queue_len--;
>>                 }
>>
>> I am wondering what kind of "lost race with folio_put()" can be.
>>
>> My understanding is that it's not necessary to handle this case here
>> because free_transhuge_page() will handle it once folio get zero ref.
>> But I must miss something here. Thanks.
> 
> free_transhuge_page() got when refcount is already zero. Both
> deferred_split_scan() and free_transhuge_page() can see the page with zero
> refcount. The check makes deferred_split_scan() to leave the page to the
> free_transhuge_page().
> 
If deferred_split_scan() leaves the page to free_transhuge_page(), is it
necessary to do
        list_del_init(&folio->_deferred_list);
        ds_queue->split_queue_len--;

Can these two line be left to free_transhuge_page() either? Thanks.

Regards
Yin, Fengwei