Re: [PATCH RFC net-next/mm V3 1/2] page_pool: Remove workqueue in new shutdown scheme

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On 28/04/2023 23.38, Toke Høiland-Jørgensen wrote:
Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes:

This removes the workqueue scheme that periodically tests when
inflight reach zero such that page_pool memory can be freed.

This change adds code to fast-path free checking for a shutdown flags
bit after returning PP pages.

Performance is very important for PP, as the fast path is used for
XDP_DROP use-cases where NIC drivers recycle PP pages directly into PP
alloc cache.

This patch (since V3) shows zero impact on this fast path. Micro
benchmarked with [1] on Intel CPU E5-1650 @3.60GHz. The slight code
reorg of likely() are deliberate.

Oh, you managed to get rid of the small difference you were seeing
before? Nice! :)

Yes! - fast path is not affected at all!!! :-)
This is due to this small change in the V3.

  +	if (page && pool->p.flags & PP_FLAG_SHUTDOWN)
  +		page_pool_empty_ring(pool);

Due to V3 change of moving free_attempt to page_pool_release_page()
the page_pool_empty_ring() call (in page_pool_put_defragged_page)
should only be called when (page != NULL).  This moves the ASM code
for the PP_FLAG_SHUTDOWN out of the fast-path.

I also measure the slightly slower path, recycle into ptr_ring, where I
cannot really see any effect.  It is easier to understand, why/when is
it important, when looking at the percentage effect of the measured 0.3
nanosec overhead. For fast-path this would be approx 7%, while for
this recycle path it is approx 1.8%.



Just a few questions, see below:

[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/bench_page_pool_simple.c

Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
  include/net/page_pool.h |    9 +--
  net/core/page_pool.c    |  138 ++++++++++++++++++++++++++++++++++-------------
  2 files changed, 103 insertions(+), 44 deletions(-)

diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index c8ec2f34722b..a71c0f2695b0 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h
@@ -50,6 +50,9 @@
  				 PP_FLAG_DMA_SYNC_DEV |\
  				 PP_FLAG_PAGE_FRAG)
+/* Internal flag: PP in shutdown phase, waiting for inflight pages */
+#define PP_FLAG_SHUTDOWN	BIT(8)
+
  /*
   * Fast allocation side cache array/stack
   *
@@ -151,11 +154,6 @@ static inline u64 *page_pool_ethtool_stats_get(u64 *data, void *stats)
  struct page_pool {
  	struct page_pool_params p;
- struct delayed_work release_dw;
-	void (*disconnect)(void *);
-	unsigned long defer_start;
-	unsigned long defer_warn;
-
  	u32 pages_state_hold_cnt;
  	unsigned int frag_offset;
  	struct page *frag_page;
@@ -165,6 +163,7 @@ struct page_pool {
  	/* these stats are incremented while in softirq context */
  	struct page_pool_alloc_stats alloc_stats;
  #endif
+	void (*disconnect)(void *);
  	u32 xdp_mem_id;
/*
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index e212e9d7edcb..54bdd140b7bd 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -23,9 +23,6 @@
#include <trace/events/page_pool.h> -#define DEFER_TIME (msecs_to_jiffies(1000))
-#define DEFER_WARN_INTERVAL (60 * HZ)
-
  #define BIAS_MAX	LONG_MAX
#ifdef CONFIG_PAGE_POOL_STATS
@@ -380,6 +377,10 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
  	struct page *page;
  	int i, nr_pages;
+ /* API usage BUG: PP in shutdown phase, cannot alloc new pages */
+	if (WARN_ON(pool->p.flags & PP_FLAG_SHUTDOWN))
+		return NULL;
+
  	/* Don't support bulk alloc for high-order pages */
  	if (unlikely(pp_order))
  		return __page_pool_alloc_page_order(pool, gfp);
@@ -450,10 +451,9 @@ EXPORT_SYMBOL(page_pool_alloc_pages);
   */
  #define _distance(a, b)	(s32)((a) - (b))
-static s32 page_pool_inflight(struct page_pool *pool)
+static s32 __page_pool_inflight(struct page_pool *pool,
+				u32 hold_cnt, u32 release_cnt)
  {
-	u32 release_cnt = atomic_read(&pool->pages_state_release_cnt);
-	u32 hold_cnt = READ_ONCE(pool->pages_state_hold_cnt);
  	s32 inflight;
inflight = _distance(hold_cnt, release_cnt);
@@ -464,6 +464,17 @@ static s32 page_pool_inflight(struct page_pool *pool)
  	return inflight;
  }
+static s32 page_pool_inflight(struct page_pool *pool)
+{
+	u32 hold_cnt = READ_ONCE(pool->pages_state_hold_cnt);
+	u32 release_cnt = atomic_read(&pool->pages_state_release_cnt);
+	return __page_pool_inflight(pool, hold_cnt, release_cnt);
+}
+
+static int page_pool_free_attempt(struct page_pool *pool,
+				  u32 hold_cnt, u32 release_cnt);
+static u32 pp_read_hold_cnt(struct page_pool *pool);
+
  /* Disconnects a page (from a page_pool).  API users can have a need
   * to disconnect a page (from a page_pool), to allow it to be used as
   * a regular page (that will eventually be returned to the normal
@@ -471,8 +482,10 @@ static s32 page_pool_inflight(struct page_pool *pool)
   */
  void page_pool_release_page(struct page_pool *pool, struct page *page)
  {
+	unsigned int flags = READ_ONCE(pool->p.flags);
  	dma_addr_t dma;
-	int count;
+	u32 release_cnt;
+	u32 hold_cnt;
if (!(pool->p.flags & PP_FLAG_DMA_MAP))
  		/* Always account for inflight pages, even if we didn't
@@ -490,11 +503,15 @@ void page_pool_release_page(struct page_pool *pool, struct page *page)
  skip_dma_unmap:
  	page_pool_clear_pp_info(page);
- /* This may be the last page returned, releasing the pool, so
-	 * it is not safe to reference pool afterwards.
-	 */
-	count = atomic_inc_return_relaxed(&pool->pages_state_release_cnt);
-	trace_page_pool_state_release(pool, page, count);
+	if (flags & PP_FLAG_SHUTDOWN)
+		hold_cnt = pp_read_hold_cnt(pool);
+
+	release_cnt = atomic_inc_return(&pool->pages_state_release_cnt);
+	trace_page_pool_state_release(pool, page, release_cnt);
+
+	/* In shutdown phase, last page will free pool instance */
+	if (flags & PP_FLAG_SHUTDOWN)
+		page_pool_free_attempt(pool, hold_cnt, release_cnt);

I'm curious why you decided to keep the hold_cnt read separate from the
call to free attempt? Not a huge deal, and I'm fine with keeping it this
way, just curious if you have any functional reason that I missed, or if
you just prefer this style? :)

You seem to have missed my explanation in V2 reply:

https://lore.kernel.org/all/f671f5da-d9bc-a559-2120-10c3491e6f6d@xxxxxxxxxx/


  }
  EXPORT_SYMBOL(page_pool_release_page);
@@ -535,7 +552,7 @@ static bool page_pool_recycle_in_ring(struct page_pool *pool, struct page *page)
  static bool page_pool_recycle_in_cache(struct page *page,
  				       struct page_pool *pool)
  {
-	if (unlikely(pool->alloc.count == PP_ALLOC_CACHE_SIZE)) {
+	if (pool->alloc.count == PP_ALLOC_CACHE_SIZE) {
  		recycle_stat_inc(pool, cache_full);
  		return false;
  	}
@@ -546,6 +563,8 @@ static bool page_pool_recycle_in_cache(struct page *page,
  	return true;
  }
+static void page_pool_empty_ring(struct page_pool *pool);
+
  /* If the page refcnt == 1, this will try to recycle the page.
   * if PP_FLAG_DMA_SYNC_DEV is set, we'll try to sync the DMA area for
   * the configured size min(dma_sync_size, pool->max_len).
@@ -572,7 +591,8 @@ __page_pool_put_page(struct page_pool *pool, struct page *page,
  			page_pool_dma_sync_for_device(pool, page,
  						      dma_sync_size);
- if (allow_direct && in_softirq() &&
+		/* During PP shutdown, no direct recycle must occur */
+		if (likely(allow_direct && in_softirq()) &&
  		    page_pool_recycle_in_cache(page, pool))
  			return NULL;
@@ -609,6 +629,8 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,
  		recycle_stat_inc(pool, ring_full);
  		page_pool_return_page(pool, page);
  	}
+	if (page && pool->p.flags & PP_FLAG_SHUTDOWN)
+		page_pool_empty_ring(pool);
  }
  EXPORT_SYMBOL(page_pool_put_defragged_page);
@@ -646,6 +668,9 @@ void page_pool_put_page_bulk(struct page_pool *pool, void **data,
  	recycle_stat_add(pool, ring, i);
  	page_pool_ring_unlock(pool);
+ if (pool->p.flags & PP_FLAG_SHUTDOWN)
+		page_pool_empty_ring(pool);
+
  	/* Hopefully all pages was return into ptr_ring */
  	if (likely(i == bulk_len))
  		return;
@@ -737,12 +762,18 @@ struct page *page_pool_alloc_frag(struct page_pool *pool,
  }
  EXPORT_SYMBOL(page_pool_alloc_frag);
+noinline
  static void page_pool_empty_ring(struct page_pool *pool)
  {
-	struct page *page;
+	struct page *page, *next;
+
+	next = ptr_ring_consume_bh(&pool->ring);
/* Empty recycle ring */
-	while ((page = ptr_ring_consume_bh(&pool->ring))) {
+	while (next) {
+		page = next;
+		next = ptr_ring_consume_bh(&pool->ring);
+
  		/* Verify the refcnt invariant of cached pages */
  		if (!(page_ref_count(page) == 1))
  			pr_crit("%s() page_pool refcnt %d violation\n",
@@ -796,39 +827,36 @@ static void page_pool_scrub(struct page_pool *pool)
  	page_pool_empty_ring(pool);
  }
-static int page_pool_release(struct page_pool *pool)
+/* Avoid inlining code to avoid speculative fetching cacheline */
+noinline
+static u32 pp_read_hold_cnt(struct page_pool *pool)
+{
+	return READ_ONCE(pool->pages_state_hold_cnt);
+}
+
+noinline
+static int page_pool_free_attempt(struct page_pool *pool,
+				  u32 hold_cnt, u32 release_cnt)
  {
  	int inflight;
- page_pool_scrub(pool);
-	inflight = page_pool_inflight(pool);
+	inflight = __page_pool_inflight(pool, hold_cnt, release_cnt);
  	if (!inflight)
  		page_pool_free(pool);
return inflight;
  }
-static void page_pool_release_retry(struct work_struct *wq)
+static int page_pool_release(struct page_pool *pool)
  {
-	struct delayed_work *dwq = to_delayed_work(wq);
-	struct page_pool *pool = container_of(dwq, typeof(*pool), release_dw);
  	int inflight;
- inflight = page_pool_release(pool);
+	page_pool_scrub(pool);
+	inflight = page_pool_inflight(pool);
  	if (!inflight)
-		return;
-
-	/* Periodic warning */
-	if (time_after_eq(jiffies, pool->defer_warn)) {
-		int sec = (s32)((u32)jiffies - (u32)pool->defer_start) / HZ;
-
-		pr_warn("%s() stalled pool shutdown %d inflight %d sec\n",
-			__func__, inflight, sec);
-		pool->defer_warn = jiffies + DEFER_WARN_INTERVAL;
-	}
+		page_pool_free(pool);
- /* Still not ready to be disconnected, retry later */
-	schedule_delayed_work(&pool->release_dw, DEFER_TIME);
+	return inflight;
  }
void page_pool_use_xdp_mem(struct page_pool *pool, void (*disconnect)(void *),
@@ -856,6 +884,10 @@ EXPORT_SYMBOL(page_pool_unlink_napi);
void page_pool_destroy(struct page_pool *pool)
  {
+	unsigned int flags;
+	u32 release_cnt;
+	u32 hold_cnt;
+
  	if (!pool)
  		return;
@@ -868,11 +900,39 @@ void page_pool_destroy(struct page_pool *pool)
  	if (!page_pool_release(pool))
  		return;
- pool->defer_start = jiffies;
-	pool->defer_warn  = jiffies + DEFER_WARN_INTERVAL;
+	/* PP have pages inflight, thus cannot immediately release memory.
+	 * Enter into shutdown phase, depending on remaining in-flight PP
+	 * pages to trigger shutdown process (on concurrent CPUs) and last
+	 * page will free pool instance.
+	 *
+	 * There exist two race conditions here, we need to take into
+	 * account in the following code.
+	 *
+	 * 1. Before setting PP_FLAG_SHUTDOWN another CPU released the last
+	 *    pages into the ptr_ring.  Thus, it missed triggering shutdown
+	 *    process, which can then be stalled forever.
+	 *
+	 * 2. After setting PP_FLAG_SHUTDOWN another CPU released the last
+	 *    page, which triggered shutdown process and freed pool
+	 *    instance. Thus, its not safe to dereference *pool afterwards.
+	 *
+	 * Handling races by holding a fake in-flight count, via
+	 * artificially bumping pages_state_hold_cnt, which assures pool
+	 * isn't freed under us.  For race(1) its safe to recheck ptr_ring
+	 * (it will not free pool). Race(2) cannot happen, and we can
+	 * release fake in-flight count as last step.
+	 */
+	hold_cnt = READ_ONCE(pool->pages_state_hold_cnt) + 1;
+	smp_store_release(&pool->pages_state_hold_cnt, hold_cnt);
+	barrier();
+	flags = READ_ONCE(pool->p.flags) | PP_FLAG_SHUTDOWN;
+	smp_store_release(&pool->p.flags, flags);

So in the memory barrier documentation, store_release() is usually
paired with read_acquire(), but the code reading the flag uses
READ_ONCE(). I'm not sure if those are equivalent? (As in, I am asking
more than I'm saying they're not; I find it difficult to keep these
things straight...)


I was hoping someone could help us say what barrier to use here?

--Jesper




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