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Date: Fri, 12 Mar 2021 11:44:09 -0800
From: Alexander Duyck <alexander.duyck@...il.com>
To: Mel Gorman <mgorman@...hsingularity.net>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Chuck Lever <chuck.lever@...cle.com>,
Jesper Dangaard Brouer <brouer@...hat.com>,
Christoph Hellwig <hch@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
Linux-Net <netdev@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
Linux-NFS <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH 7/7] net: page_pool: use alloc_pages_bulk in refill code path
On Fri, Mar 12, 2021 at 7:43 AM Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> From: Jesper Dangaard Brouer <brouer@...hat.com>
>
> There are cases where the page_pool need to refill with pages from the
> page allocator. Some workloads cause the page_pool to release pages
> instead of recycling these pages.
>
> For these workload it can improve performance to bulk alloc pages from
> the page-allocator to refill the alloc cache.
>
> For XDP-redirect workload with 100G mlx5 driver (that use page_pool)
> redirecting xdp_frame packets into a veth, that does XDP_PASS to create
> an SKB from the xdp_frame, which then cannot return the page to the
> page_pool. In this case, we saw[1] an improvement of 18.8% from using
> the alloc_pages_bulk API (3,677,958 pps -> 4,368,926 pps).
>
> [1] https://github.com/xdp-project/xdp-project/blob/master/areas/mem/page_pool06_alloc_pages_bulk.org
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> Reviewed-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
> ---
> net/core/page_pool.c | 62 ++++++++++++++++++++++++++++----------------
> 1 file changed, 39 insertions(+), 23 deletions(-)
>
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 40e1b2beaa6c..a5889f1b86aa 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -208,44 +208,60 @@ noinline
> static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
> gfp_t _gfp)
> {
> + const int bulk = PP_ALLOC_CACHE_REFILL;
> + struct page *page, *next, *first_page;
> unsigned int pp_flags = pool->p.flags;
> - struct page *page;
> + unsigned int pp_order = pool->p.order;
> + int pp_nid = pool->p.nid;
> + LIST_HEAD(page_list);
> gfp_t gfp = _gfp;
>
> - /* We could always set __GFP_COMP, and avoid this branch, as
> - * prep_new_page() can handle order-0 with __GFP_COMP.
> - */
> - if (pool->p.order)
> + /* Don't support bulk alloc for high-order pages */
> + if (unlikely(pp_order)) {
> gfp |= __GFP_COMP;
> + first_page = alloc_pages_node(pp_nid, gfp, pp_order);
> + if (unlikely(!first_page))
> + return NULL;
> + goto out;
> + }
>
> - /* FUTURE development:
> - *
> - * Current slow-path essentially falls back to single page
> - * allocations, which doesn't improve performance. This code
> - * need bulk allocation support from the page allocator code.
> - */
> -
> - /* Cache was empty, do real allocation */
> -#ifdef CONFIG_NUMA
> - page = alloc_pages_node(pool->p.nid, gfp, pool->p.order);
> -#else
> - page = alloc_pages(gfp, pool->p.order);
> -#endif
> - if (!page)
> + if (unlikely(!__alloc_pages_bulk(gfp, pp_nid, NULL, bulk, &page_list)))
> return NULL;
>
> + /* First page is extracted and returned to caller */
> + first_page = list_first_entry(&page_list, struct page, lru);
> + list_del(&first_page->lru);
> +
This seems kind of broken to me. If you pull the first page and then
cannot map it you end up returning NULL even if you placed a number of
pages in the cache.
It might make more sense to have the loop below record a pointer to
the last page you processed and handle things in two stages so that on
the first iteration you map one page.
So something along the lines of:
1. Initialize last_page to NULL
for each page in the list
2. Map page
3. If last_page is non-NULL, move to cache
4. Assign page to last_page
5. Return to step 2 for each page in list
6. return last_page
> + /* Remaining pages store in alloc.cache */
> + list_for_each_entry_safe(page, next, &page_list, lru) {
> + list_del(&page->lru);
> + if ((pp_flags & PP_FLAG_DMA_MAP) &&
> + unlikely(!page_pool_dma_map(pool, page))) {
> + put_page(page);
> + continue;
> + }
So if you added a last_page pointer what you could do is check for it
here and assign it to the alloc cache. If last_page is not set the
block would be skipped.
> + if (likely(pool->alloc.count < PP_ALLOC_CACHE_SIZE)) {
> + pool->alloc.cache[pool->alloc.count++] = page;
> + pool->pages_state_hold_cnt++;
> + trace_page_pool_state_hold(pool, page,
> + pool->pages_state_hold_cnt);
> + } else {
> + put_page(page);
If you are just calling put_page here aren't you leaking DMA mappings?
Wouldn't you need to potentially unmap the page before you call
put_page on it?
> + }
> + }
> +out:
> if ((pp_flags & PP_FLAG_DMA_MAP) &&
> - unlikely(!page_pool_dma_map(pool, page))) {
> - put_page(page);
> + unlikely(!page_pool_dma_map(pool, first_page))) {
> + put_page(first_page);
I would probably move this block up and make it a part of the pp_order
block above. Also since you are doing this in 2 spots it might make
sense to look at possibly making this an inline function.
> return NULL;
> }
>
> /* Track how many pages are held 'in-flight' */
> pool->pages_state_hold_cnt++;
> - trace_page_pool_state_hold(pool, page, pool->pages_state_hold_cnt);
> + trace_page_pool_state_hold(pool, first_page, pool->pages_state_hold_cnt);
>
> /* When page just alloc'ed is should/must have refcnt 1. */
> - return page;
> + return first_page;
> }
>
> /* For using page_pool replace: alloc_pages() API calls, but provide
> --
> 2.26.2
>
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