linux-kernel - Re: [PATCH net-next v5 3/5] page_pool: Allow drivers to hint on SKB recycling

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Message-ID: <YKJVEDUjmv6rRnFP@enceladus>
Date:   Mon, 17 May 2021 14:35:44 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Matteo Croce <mcroce@...ux.microsoft.com>, netdev@...r.kernel.org,
        linux-mm@...ck.org, Ayush Sawal <ayush.sawal@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Russell King <linux@...linux.org.uk>,
        Mirko Lindner <mlindner@...vell.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Boris Pismenny <borisp@...dia.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Will Deacon <will@...nel.org>,
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        Guillaume Nault <gnault@...hat.com>,
        linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
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        Andrew Lunn <andrew@...n.ch>, Paolo Abeni <pabeni@...hat.com>,
        Sven Auhagen <sven.auhagen@...eatech.de>
Subject: Re: [PATCH net-next v5 3/5] page_pool: Allow drivers to hint on SKB
 recycling

On Mon, May 17, 2021 at 07:10:09PM +0800, Yunsheng Lin wrote:
> On 2021/5/17 17:36, Ilias Apalodimas wrote:
>  >>
> >> Even if when skb->pp_recycle is 1, pages allocated from page allocator directly
> >> or page pool are both supported, so it seems page->signature need to be reliable
> >> to indicate a page is indeed owned by a page pool, which means the skb->pp_recycle
> >> is used mainly to short cut the code path for skb->pp_recycle is 0 case, so that
> >> the page->signature does not need checking?
> > 
> > Yes, the idea for the recycling bit, is that you don't have to fetch the page
> > in cache do do more processing (since freeing is asynchronous and we
> > can't have any guarantees on what the cache will have at that point).  So we
> > are trying to affect the existing release path a less as possible. However it's
> > that new skb bit that triggers the whole path.
> > 
> > What you propose could still be doable though.  As you said we can add the
> > page pointer to struct page when we allocate a page_pool page and never
> > reset it when we recycle the buffer. But I don't think there will be any
> > performance impact whatsoever. So I prefer the 'visible' approach, at least for
> 
> setting and unsetting the page_pool ptr every time the page is recycled may
> cause a cache bouncing problem when rx cleaning and skb releasing is not
> happening on the same cpu.

In our case since the skb is asynchronous and not protected by a NAPI context,
the buffer wont end up in the 'fast' page pool cache.  So we'll recycle by
calling page_pool_recycle_in_ring() not page_pool_recycle_in_cache().  Which
means that the page you recycled will be re-filled later, in batches, when
page_pool_refill_alloc_cache() is called to refill the fast cache.  I am not i
saying it might not happen, but I don't really know if it's going to make a
difference or not.  So I just really prefer taking this as is and perhaps
later, when 40/100gbit drivers start using it we can justify the optimization
(along with supporting the split page model).

Thanks
/Ilias

> 
> > the first iteration.
> > 
> > Thanks
> > /Ilias
> >  
> > 
> > .
> > 
>