[RFC PATCH net-next] page_pool: Add page_pool_release_stalled tracepoint

[Leon Hwang <leon.hwang@linux.dev>]

Introduce a new tracepoint to track stalled page pool releases,
providing better observability for page pool lifecycle issues.

Problem:
Currently, when a page pool shutdown is stalled due to inflight pages,
the kernel only logs a warning message via pr_warn(). This has several
limitations:

1. The warning floods the kernel log after the initial DEFER_WARN_INTERVAL,
   making it difficult to track the progression of stalled releases
2. There's no structured way to monitor or analyze these events
3. Debugging tools cannot easily capture and correlate stalled pool
   events with other network activity

Solution:
Add a new tracepoint, page_pool_release_stalled, that fires when a page
pool shutdown is stalled. The tracepoint captures:
- pool: pointer to the stalled page_pool
- inflight: number of pages still in flight
- sec: seconds since the release was deferred

The implementation also modifies the logging behavior:
- pr_warn() is only emitted during the first warning interval
  (DEFER_WARN_INTERVAL to DEFER_WARN_INTERVAL*2)
- The tracepoint is fired always, reducing log noise while still
  allowing monitoring tools to track the issue

This allows developers and system administrators to:
- Use tools like perf, ftrace, or eBPF to monitor stalled releases
- Correlate page pool issues with network driver behavior
- Analyze patterns without parsing kernel logs
- Track the progression of inflight page counts over time

Signed-off-by: Leon Huang Fu <leon.huangfu@shopee.com>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 include/trace/events/page_pool.h | 22 ++++++++++++++++++++++
 net/core/page_pool.c             |  6 ++++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/include/trace/events/page_pool.h b/include/trace/events/page_pool.h
index 31825ed30032..c4205af9bf93 100644
--- a/include/trace/events/page_pool.h
+++ b/include/trace/events/page_pool.h
@@ -113,6 +113,28 @@ TRACE_EVENT(page_pool_update_nid,
 		  __entry->pool, __entry->pool_nid, __entry->new_nid)
 );
 
+TRACE_EVENT(page_pool_release_stalled,
+
+	TP_PROTO(const struct page_pool *pool, int inflight, int sec),
+
+	TP_ARGS(pool, inflight, sec),
+
+	TP_STRUCT__entry(
+		__field(const struct page_pool *, pool)
+		__field(int,			  inflight)
+		__field(int,			  sec)
+	),
+
+	TP_fast_assign(
+		__entry->pool		= pool;
+		__entry->inflight	= inflight;
+		__entry->sec		= sec;
+	),
+
+	TP_printk("page_pool=%p id=%d inflight=%d sec=%d",
+		  __entry->pool, __entry->pool->user.id, __entry->inflight, __entry->sec)
+);
+
 #endif /* _TRACE_PAGE_POOL_H */
 
 /* This part must be outside protection */
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 1a5edec485f1..9fd86749c705 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -1218,8 +1218,10 @@ static void page_pool_release_retry(struct work_struct *wq)
 	    (!netdev || netdev == NET_PTR_POISON)) {
 		int sec = (s32)((u32)jiffies - (u32)pool->defer_start) / HZ;
 
-		pr_warn("%s() stalled pool shutdown: id %u, %d inflight %d sec\n",
-			__func__, pool->user.id, inflight, sec);
+		if (sec >= DEFER_WARN_INTERVAL / HZ && sec < DEFER_WARN_INTERVAL * 2 / HZ)
+			pr_warn("%s() stalled pool shutdown: id %u, %d inflight %d sec\n",
+				__func__, pool->user.id, inflight, sec);
+		trace_page_pool_release_stalled(pool, inflight, sec);
 		pool->defer_warn = jiffies + DEFER_WARN_INTERVAL;
 	}
 
-- 
2.52.0