Re: [PATCH net-next v6 8/8] page_pool: use list instead of array for alloc cache

On 2025/1/7 20:03, Simon Horman wrote:

On Mon, Jan 06, 2025 at 09:01:16PM +0800, Yunsheng Lin wrote:

quoted

As the alloc cache is always protected by NAPI context
protection, use encoded_next as a pointer to a next item
to avoid the using the array.

Testing shows there is about 3ns improvement for the
performance of 'time_bench_page_pool01_fast_path' test
case.

CC: Robin Murphy <robin.murphy@arm.com>
CC: Alexander Duyck <redacted>
CC: IOMMU <redacted>
Signed-off-by: Yunsheng Lin <redacted>

...

quoted

diff --git a/net/core/page_pool.c b/net/core/page_pool.c

...

quoted

@@ -677,10 +698,12 @@ static void __page_pool_return_page(struct page_pool *pool, netmem_ref netmem,
 
 static noinline netmem_ref page_pool_refill_alloc_cache(struct page_pool *pool)
 {
-	struct page_pool_item *refill;
+	struct page_pool_item *refill, *alloc, *curr;
 	netmem_ref netmem;
 	int pref_nid; /* preferred NUMA node */
 
+	DEBUG_NET_WARN_ON_ONCE(pool->alloc.count || pool->alloc.list);
+
 	/* Quicker fallback, avoid locks when ring is empty */
 	refill = pool->alloc.refill;
 	if (unlikely(!refill && !READ_ONCE(pool->ring.list))) {

The checking has ensured that netmem will be initialised below.

quoted

@@ -698,6 +721,7 @@ static noinline netmem_ref page_pool_refill_alloc_cache(struct page_pool *pool)
 	pref_nid = numa_mem_id(); /* will be zero like page_to_nid() */
 #endif
 
+	alloc = NULL;
 	/* Refill alloc array, but only if NUMA match */
 	do {
 		if (unlikely(!refill)) {

@@ -706,10 +730,13 @@ static noinline netmem_ref page_pool_refill_alloc_cache(struct page_pool *pool)
 				break;
 		}
 
+		curr = refill;
 		netmem = refill->pp_netmem;

initialised here.

quoted

 		refill = page_pool_item_get_next(refill);
 		if (likely(netmem_is_pref_nid(netmem, pref_nid))) {
-			pool->alloc.cache[pool->alloc.count++] = netmem;
+			page_pool_item_set_next(curr, alloc);
+			pool->alloc.count++;
+			alloc = curr;
 		} else {
 			/* NUMA mismatch;
 			 * (1) release 1 page to page-allocator and

And set to zero here.

quoted

@@ -729,7 +756,8 @@ static noinline netmem_ref page_pool_refill_alloc_cache(struct page_pool *pool)
 	/* Return last page */
 	if (likely(pool->alloc.count > 0)) {
 		atomic_sub(pool->alloc.count, &pool->ring.count);
-		netmem = pool->alloc.cache[--pool->alloc.count];

So we still need to set netmem to something meaningful here if netmem
is set to zero in the above 'else' branch.

quoted

+		pool->alloc.list = page_pool_item_get_next(alloc);
+		pool->alloc.count--;
 		alloc_stat_inc(pool, refill);
 	}
 

Hi Yunsheng Lin,

The following line of the code looks like this:

	return netmem;

And, with this patch applied, Smatch warns that netmem may be used
uninitialised here. I assume this is because it is no longer conditionally
initialised above.

Thanks for reminding, the tool does seem to catch some bug here.

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