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

[Alexander Duyck]

On Sat, Nov 10, 2018 at 3:54 PM PaweÅ Staszewski <pstaszewski@xxxxxxxxx> wrote:
>
>
>
> W dniu 05.11.2018 o 16:44, Alexander Duyck pisze:
> > On Mon, Nov 5, 2018 at 12:58 AM Aaron Lu <aaron.lu@xxxxxxxxx> 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()
> >>      -> 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.
> >>
> >> [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>
> >> Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
> >> ---
> >>   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...

The problem is if you are going for less interrupts you are setting
yourself up for buffer bloat. Basically you are going to use much more
cache and much more memory then you actually need and if things are
properly configured NAPI should take care of the interrupts anyway
since under maximum load you shouldn't stop polling normally.

One issue I have seen is people delay interrupts for as long as
possible which isn't really a good thing since most network
controllers will use NAPI which will disable the interrupts and leave
them disabled whenever the system is under heavy stress so you should
be able to get the maximum performance by configuring an adapter with
small ring sizes and for high interrupt rates.

It is easiest to think of it this way. Your total packet rate is equal
to your interrupt rate times the number of buffers you will store in
the ring. So if you have some fixed rate "X" for packets and an
interrupt rate of "i" then your optimal ring size should be "X/i". So
if you lower the interrupt rate you end up hurting the throughput
unless you increase the buffer size. However at a certain point the
buffer size starts becoming an issue. For example with UDP flows I
often see massive packet drops if you tune the interrupt rate too low
and then put the system under heavy stress.

- Alex