Re: [PATCH net-next 3/4] page_pool: Restructure __page_pool_put_page()

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Wed, 23 Oct 2019 11:45:15 +0300
Ilias Apalodimas <ilias.apalodimas@linaro.org> wrote:

> On Tue, Oct 22, 2019 at 04:44:24AM +0000, Saeed Mahameed wrote:
> > From: Jonathan Lemon <jonathan.lemon@gmail.com>
> > 
> > 1) Rename functions to reflect what they are actually doing.
> > 
> > 2) Unify the condition to keep a page.
> > 
> > 3) When page can't be kept in cache, fallback to releasing page to page
> > allocator in one place, instead of calling it from multiple conditions,
> > and reuse __page_pool_return_page().
> > 
> > Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
> > Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
> > ---
> >  net/core/page_pool.c | 38 +++++++++++++++++++-------------------
> >  1 file changed, 19 insertions(+), 19 deletions(-)
> > 
> > diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> > index 8120aec999ce..65680aaa0818 100644
> > --- a/net/core/page_pool.c
> > +++ b/net/core/page_pool.c
> > @@ -258,6 +258,7 @@ static bool __page_pool_recycle_into_ring(struct page_pool *pool,
> >  				   struct page *page)
> >  {
> >  	int ret;
> > +
> >  	/* BH protection not needed if current is serving softirq */
> >  	if (in_serving_softirq())
> >  		ret = ptr_ring_produce(&pool->ring, page);
> > @@ -272,8 +273,8 @@ static bool __page_pool_recycle_into_ring(struct page_pool *pool,
> >   *
> >   * Caller must provide appropriate safe context.
> >   */
> > -static bool __page_pool_recycle_direct(struct page *page,
> > -				       struct page_pool *pool)
> > +static bool __page_pool_recycle_into_cache(struct page *page,
> > +					   struct page_pool *pool)
> >  {
> >  	if (unlikely(pool->alloc.count == PP_ALLOC_CACHE_SIZE))
> >  		return false;
> > @@ -283,15 +284,18 @@ static bool __page_pool_recycle_direct(struct page *page,
> >  	return true;
> >  }
> >  
> > -/* page is NOT reusable when:
> > - * 1) allocated when system is under some pressure. (page_is_pfmemalloc)
> > - * 2) belongs to a different NUMA node than pool->p.nid.
> > +/* Keep page in caches only if page:
> > + * 1) wasn't allocated when system is under some pressure (page_is_pfmemalloc).
> > + * 2) belongs to pool's numa node (pool->p.nid).
> > + * 3) refcount is 1 (owned by page pool).
> >   *
> >   * To update pool->p.nid users must call page_pool_update_nid.
> >   */
> > -static bool pool_page_reusable(struct page_pool *pool, struct page *page)
> > +static bool page_pool_keep_page(struct page_pool *pool, struct page *page)
> >  {
> > -	return !page_is_pfmemalloc(page) && page_to_nid(page) == pool->p.nid;
> > +	return !page_is_pfmemalloc(page) &&
> > +	       page_to_nid(page) == pool->p.nid &&
> > +	       page_ref_count(page) == 1;
> >  }
> >  
> >  void __page_pool_put_page(struct page_pool *pool,
> > @@ -300,22 +304,19 @@ void __page_pool_put_page(struct page_pool *pool,
> >  	/* This allocator is optimized for the XDP mode that uses
> >  	 * one-frame-per-page, but have fallbacks that act like the
> >  	 * regular page allocator APIs.
> > -	 *
> > -	 * refcnt == 1 means page_pool owns page, and can recycle it.
> >  	 */
> > -	if (likely(page_ref_count(page) == 1 &&
> > -		   pool_page_reusable(pool, page))) {
> > +
> > +	if (likely(page_pool_keep_page(pool, page))) {
> >  		/* Read barrier done in page_ref_count / READ_ONCE */
> >  
> >  		if (allow_direct && in_serving_softirq())
> > -			if (__page_pool_recycle_direct(page, pool))
> > +			if (__page_pool_recycle_into_cache(page, pool))
> >  				return;
> >  
> > -		if (!__page_pool_recycle_into_ring(pool, page)) {
> > -			/* Cache full, fallback to free pages */
> > -			__page_pool_return_page(pool, page);
> > -		}
> > -		return;
> > +		if (__page_pool_recycle_into_ring(pool, page))
> > +			return;
> > +
> > +		/* Cache full, fallback to return pages */
> >  	}
> >  	/* Fallback/non-XDP mode: API user have elevated refcnt.
> >  	 *
> > @@ -330,8 +331,7 @@ void __page_pool_put_page(struct page_pool *pool,
> >  	 * doing refcnt based recycle tricks, meaning another process
> >  	 * will be invoking put_page.
> >  	 */
> > -	__page_pool_clean_page(pool, page);
> > -	put_page(page);
> > +	__page_pool_return_page(pool, page);  
> 
> I think Jesper had a reason for calling them separately instead of 
> __page_pool_return_page + put_page() (which in fact does the same thing). 
> 
> In the future he was planning on removing the __page_pool_clean_page call from
> there, since someone might call __page_pool_put_page() after someone has called
> __page_pool_clean_page()

Yes.  We need to work on removing this  __page_pool_clean_page() call,
to fulfill the plans of SKB returning/recycling page_pool pages.

> Can we leave the calls there as-is?

Yes, please.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer