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. > > > > Paweł 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, Paweł 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: Paweł 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> > > ---
