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

Yin Fengwei
2023-04-26T02:08:19Z

On 4/25/23 20:38, 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?
Huang Ying pointed out the race with deferred_split_scan(). And Yes. Need
recheck under lock. Will update in next version.

Regards
Yin, Fengwei

>
>> ds_queue->split_queue_len--;
>> list_del(&folio->_deferred_list);
>> + spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>> }
>> - spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>> free_compound_page(page);
>> }
>>
>> --
>> 2.30.2
>>
>>
>