Messages in this thread | | | Subject | Re: [PATCH 1/2] mm/page_alloc: free order-0 pages through PCP in page_frag_free() | From | Paweł Staszewski <> | Date | Sun, 11 Nov 2018 00:54:27 +0100 |
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W dniu 05.11.2018 o 16:44, Alexander Duyck pisze: > On Mon, Nov 5, 2018 at 12:58 AM Aaron Lu <aaron.lu@intel.com> 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() >> -> 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. >> >> [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@itcare.pl> >> Analysed-by: Jesper Dangaard Brouer <brouer@redhat.com> >> Signed-off-by: Aaron Lu <aaron.lu@intel.com> >> --- >> mm/page_alloc.c | 10 ++++++++-- >> 1 file changed, 8 insertions(+), 2 deletions(-) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index ae31839874b8..91a9a6af41a2 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -4555,8 +4555,14 @@ void page_frag_free(void *addr) >> { >> struct page *page = virt_to_head_page(addr); >> >> - if (unlikely(put_page_testzero(page))) >> - __free_pages_ok(page, compound_order(page)); >> + if (unlikely(put_page_testzero(page))) { >> + unsigned int order = compound_order(page); >> + >> + if (order == 0) >> + free_unref_page(page); >> + else >> + __free_pages_ok(page, order); >> + } >> } >> EXPORT_SYMBOL(page_frag_free); >> > One thing I would suggest for Pawel to try would be to reduce the Tx > qdisc size on his transmitting interfaces, Reduce the Tx ring size, > and possibly increase the Tx interrupt rate. Ideally we shouldn't have > too many packets in-flight and I suspect that is the issue that Pawel > is seeing that is leading to the page pool allocator freeing up the > memory. I know we like to try to batch things but the issue is > processing too many Tx buffers in one batch leads to us eating up too > much memory and causing evictions from the cache. Ideally the Rx and > Tx rings and queues should be sized as small as possible while still > allowing us to process up to our NAPI budget. Usually I run things > with a 128 Rx / 128 Tx setup and then reduce the Tx queue length so we > don't have more buffers stored there than we can place in the Tx ring. > Then we can avoid the extra thrash of having to pull/push memory into > and out of the freelists. Essentially the issue here ends up being > another form of buffer bloat. Thanks Aleksandar - yes it can be - but in my scenario setting RX buffer <4096 producing more interface rx drops - and no_rx_buffer on network controller that is receiving more packets So i need to stick with 3000-4000 on RX - and yes i was trying to lower the TX buff on connectx4 - but that changed nothing before Aaron patch
After Aaron patch - decreasing TX buffer influencing total bandwidth that can be handled by the router/server Dono why before this patch there was no difference there no matter what i set there there was always page_alloc/slowpath on top in perf
Currently testing RX4096/TX256 - this helps with bandwidth like +10% more bandwidth with less interrupts...
> > With that said this change should be mostly harmless and does address > the fact that we can have both regular order 0 pages and page frags > used for skb->head. > > Acked-by: Alexander Duyck <alexander.h.duyck@linux.intel.com> >
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