[PATCH V2] mm/cma: using per-CMA locks to improve concurrent allocation performance

[yangge1116@126.com]

From: yangge <yangge1116@126.com>

For different CMAs, concurrent allocation of CMA memory ideally should not
require synchronization using locks. Currently, a global cma_mutex lock is
employed to synchronize all CMA allocations, which can impact the
performance of concurrent allocations across different CMAs.

To test the performance impact, follow these steps:
1. Boot the kernel with the command line argument hugetlb_cma=30G to
   allocate a 30GB CMA area specifically for huge page allocations. (note:
   on my machine, which has 3 nodes, each node is initialized with 10G of
   CMA)
2. Use the dd command with parameters if=/dev/zero of=/dev/shm/file bs=1G
   count=30 to fully utilize the CMA area by writing zeroes to a file in
   /dev/shm.
3. Open three terminals and execute the following commands simultaneously:
   (Note: Each of these commands attempts to allocate 10GB [2621440 * 4KB
   pages] of CMA memory.)
   On Terminal 1: time echo 2621440 > /sys/kernel/debug/cma/hugetlb1/alloc
   On Terminal 2: time echo 2621440 > /sys/kernel/debug/cma/hugetlb2/alloc
   On Terminal 3: time echo 2621440 > /sys/kernel/debug/cma/hugetlb3/alloc

We attempt to allocate pages through the CMA debug interface and use the
time command to measure the duration of each allocation.
Performance comparison:
             Without this patch      With this patch
Terminal1        ~7s                     ~7s
Terminal2       ~14s                     ~8s
Terminal3       ~21s                     ~7s

To slove problem above, we could use per-CMA locks to improve concurrent
allocation performance. This would allow each CMA to be managed
independently, reducing the need for a global lock and thus improving
scalability and performance.

Signed-off-by: yangge <yangge1116@126.com>
---

V2:
- update code and message suggested by Barry. 

 mm/cma.c | 7 ++++---
 mm/cma.h | 1 +
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/cma.c b/mm/cma.c
index 34a4df2..a0d4d2f 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -34,7 +34,6 @@
 
 struct cma cma_areas[MAX_CMA_AREAS];
 unsigned int cma_area_count;
-static DEFINE_MUTEX(cma_mutex);
 
 static int __init __cma_declare_contiguous_nid(phys_addr_t base,
 			phys_addr_t size, phys_addr_t limit,
@@ -175,6 +174,8 @@ static void __init cma_activate_area(struct cma *cma)
 
 	spin_lock_init(&cma->lock);
 
+	mutex_init(&cma->alloc_mutex);
+
 #ifdef CONFIG_CMA_DEBUGFS
 	INIT_HLIST_HEAD(&cma->mem_head);
 	spin_lock_init(&cma->mem_head_lock);
@@ -813,9 +814,9 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
 		spin_unlock_irq(&cma->lock);
 
 		pfn = cmr->base_pfn + (bitmap_no << cma->order_per_bit);
-		mutex_lock(&cma_mutex);
+		mutex_lock(&cma->alloc_mutex);
 		ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA, gfp);
-		mutex_unlock(&cma_mutex);
+		mutex_unlock(&cma->alloc_mutex);
 		if (ret == 0) {
 			page = pfn_to_page(pfn);
 			break;
diff --git a/mm/cma.h b/mm/cma.h
index df7fc62..41a3ab0 100644
--- a/mm/cma.h
+++ b/mm/cma.h
@@ -39,6 +39,7 @@ struct cma {
 	unsigned long	available_count;
 	unsigned int order_per_bit; /* Order of pages represented by one bit */
 	spinlock_t	lock;
+	struct mutex alloc_mutex;
 #ifdef CONFIG_CMA_DEBUGFS
 	struct hlist_head mem_head;
 	spinlock_t mem_head_lock;
-- 
2.7.4