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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 28 Apr 2023 23:38:19 +0200

Jesper Dangaard Brouer <[email-masked]> 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! :)

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 <[email-masked]>
> ---
>  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? :)

>  }
>  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...)

-Toke