[PATCH v3 1/2] THP: avoid lock when check whether THP is in deferred list

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

free_transhuge_page() acquires split queue lock then check
whether the THP was added to deferred list or not. It brings
high deferred queue lock contention.

It's safe to check whether the THP is in deferred list or not
without holding the deferred queue lock in free_transhuge_page()
because when code hit free_transhuge_page(), there is no one
tries to add the folio to _deferred_list.

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:
-   63.02%     0.01%  page_fault1_pro  [kernel.kallsyms]         [k] free_transhuge_page
   - 63.01% free_transhuge_page
      + 62.91% _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>
---
 mm/huge_memory.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 032fb0ef9cd1..2a1df2c24c8e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2799,12 +2799,19 @@ 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)) {
-		ds_queue->split_queue_len--;
-		list_del(&folio->_deferred_list);
+	/*
+	 * At this point, there is no one trying to add the folio to
+	 * deferred_list. If folio is not in deferred_list, it's safe
+	 * to check without acquiring the split_queue_lock.
+	 */
+	if (data_race(!list_empty(&folio->_deferred_list))) {
+		spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
+		if (!list_empty(&folio->_deferred_list)) {
+			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);
 }
 
-- 
2.34.1