netdev - Re: [PATCH net-next] page_pool: fix logic in __page_pool_get

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Message-ID: <20190814124901.GA25587@apalos>
Date:   Wed, 14 Aug 2019 15:49:01 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, brouer@...hat.com,
        saeedm@...lanox.com, ttoukan.linux@...il.com, kernel-team@...com
Subject: Re: [PATCH net-next] page_pool: fix logic in __page_pool_get_cached

Hi Jonathan,

Thanks!

On Tue, Aug 13, 2019 at 10:45:09AM -0700, Jonathan Lemon wrote:
> __page_pool_get_cached() will return NULL when the ring is
> empty, even if there are pages present in the lookaside cache.
> 
> It is also possible to refill the cache, and then return a
> NULL page.
> 
> Restructure the logic so eliminate both cases.

Acked-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>

> 
> Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
> ---
>  net/core/page_pool.c | 39 ++++++++++++++++-----------------------
>  1 file changed, 16 insertions(+), 23 deletions(-)
> 
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 68510eb869ea..de09a74a39a4 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -82,12 +82,9 @@ EXPORT_SYMBOL(page_pool_create);
>  static struct page *__page_pool_get_cached(struct page_pool *pool)
>  {
>  	struct ptr_ring *r = &pool->ring;
> +	bool refill = false;
>  	struct page *page;
>  
> -	/* Quicker fallback, avoid locks when ring is empty */
> -	if (__ptr_ring_empty(r))
> -		return NULL;
> -
>  	/* Test for safe-context, caller should provide this guarantee */
>  	if (likely(in_serving_softirq())) {
>  		if (likely(pool->alloc.count)) {
> @@ -95,27 +92,23 @@ static struct page *__page_pool_get_cached(struct page_pool *pool)
>  			page = pool->alloc.cache[--pool->alloc.count];
>  			return page;
>  		}
> -		/* Slower-path: Alloc array empty, time to refill
> -		 *
> -		 * Open-coded bulk ptr_ring consumer.
> -		 *
> -		 * Discussion: the ring consumer lock is not really
> -		 * needed due to the softirq/NAPI protection, but
> -		 * later need the ability to reclaim pages on the
> -		 * ring. Thus, keeping the locks.
> -		 */
> -		spin_lock(&r->consumer_lock);
> -		while ((page = __ptr_ring_consume(r))) {
> -			if (pool->alloc.count == PP_ALLOC_CACHE_REFILL)
> -				break;
> -			pool->alloc.cache[pool->alloc.count++] = page;
> -		}
> -		spin_unlock(&r->consumer_lock);
> -		return page;
> +		refill = true;
>  	}
>  
> -	/* Slow-path: Get page from locked ring queue */
> -	page = ptr_ring_consume(&pool->ring);
> +	/* Quicker fallback, avoid locks when ring is empty */
> +	if (__ptr_ring_empty(r))
> +		return NULL;
> +
> +	/* Slow-path: Get page from locked ring queue,
> +	 * refill alloc array if requested.
> +	 */
> +	spin_lock(&r->consumer_lock);
> +	page = __ptr_ring_consume(r);
> +	if (refill)
> +		pool->alloc.count = __ptr_ring_consume_batched(r,
> +							pool->alloc.cache,
> +							PP_ALLOC_CACHE_REFILL);
> +	spin_unlock(&r->consumer_lock);
>  	return page;
>  }
>  
> -- 
> 2.17.1
>