Re: [PATCH v2 RESEND 1/2] mm/page_alloc: free order-0 pages through PCP in page_frag_free()

[Aaron Lu]

On Mon, Nov 19, 2018 at 03:00:53PM +0000, Tariq Toukan wrote:
> 
> 
> On 19/11/2018 3:48 PM, Aaron Lu wrote:
> > page_frag_free() calls __free_pages_ok() to free the page back to
> > Buddy. This is OK for high order page, but for order-0 pages, it
> > misses the optimization opportunity of using Per-Cpu-Pages and can
> > cause zone lock contention when called frequently.
> > 
> > PaweÅ Staszewski recently shared his result of 'how Linux kernel
> > handles normal traffic'[1] and from perf data, Jesper Dangaard Brouer
> > found the lock contention comes from page allocator:
> > 
> >    mlx5e_poll_tx_cq
> >    |
> >     --16.34%--napi_consume_skb
> >               |
> >               |--12.65%--__free_pages_ok
> >               |          |
> >               |           --11.86%--free_one_page
> >               |                     |
> >               |                     |--10.10%--queued_spin_lock_slowpath
> >               |                     |
> >               |                      --0.65%--_raw_spin_lock
> >               |
> >               |--1.55%--page_frag_free
> >               |
> >                --1.44%--skb_release_data
> > 
> > Jesper explained how it happened: mlx5 driver RX-page recycle
> > mechanism is not effective in this workload and pages have to go
> > through the page allocator. The lock contention happens during
> > mlx5 DMA TX completion cycle. And the page allocator cannot keep
> > up at these speeds.[2]
> > 
> > I thought that __free_pages_ok() are mostly freeing high order
> > pages and thought this is an lock contention for high order pages
> > but Jesper explained in detail that __free_pages_ok() here are
> > actually freeing order-0 pages because mlx5 is using order-0 pages
> > to satisfy its page pool allocation request.[3]
> > 
> > The free path as pointed out by Jesper is:
> > skb_free_head()
> >    -> skb_free_frag()
> >      -> page_frag_free()
> > And the pages being freed on this path are order-0 pages.
> > 
> > Fix this by doing similar things as in __page_frag_cache_drain() -
> > send the being freed page to PCP if it's an order-0 page, or
> > directly to Buddy if it is a high order page.
> > 
> > With this change, PaweÅ hasn't noticed lock contention yet in
> > his workload and Jesper has noticed a 7% performance improvement
> > using a micro benchmark and lock contention is gone. Ilias' test
> > on a 'low' speed 1Gbit interface on an cortex-a53 shows ~11%
> > performance boost testing with 64byte packets and __free_pages_ok()
> > disappeared from perf top.
> > 
> > [1]: https://www.spinics.net/lists/netdev/msg531362.html
> > [2]: https://www.spinics.net/lists/netdev/msg531421.html
> > [3]: https://www.spinics.net/lists/netdev/msg531556.html
> > 
> > Reported-by: PaweÅ Staszewski <pstaszewski@xxxxxxxxx>
> > Analysed-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> > Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> > Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Acked-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> > Acked-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
> > Tested-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
> > Acked-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > Acked-by: Tariq Toukan <tariqt@xxxxxxxxxxxx
> 
> missing '>' sign in my email tag.

Sorry about that, will fix this and resend.
 
> > Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
> > ---