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

[PaweÅ Staszewski]

W dniu 05.11.2018 oÂ16:44, 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...


> 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@xxxxxxxxxxxxxxx>