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

["Paweł Staszewski" <pstaszewski@itcare.pl>]

W dniu 12.11.2018 o 00:05, Alexander Duyck pisze:
> On Sat, Nov 10, 2018 at 3:54 PM Paweł Staszewski <pstaszewski@itcare.pl> 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@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...
> 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.

Im trying to balance here - there is problem cause server is forwarding 
all kingd of protocols packets/different size etc

The problem is im trying to go in high interrupt rate - but

Setting coalescence to adaptative for rx killing cpu's at 22Gbit/s RX 
and 22Gbit with rly high interrupt rate

So adding a little more latency i can turn off adaptative rx and setup 
rx-usecs from range 16-64 - and this gives me more or less interrupts - 
but the problem is - always same bandwidth as maximum


>
> 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.

Sure this is bad to setup rx-usec for high values - cause at some point 
this will add high latency for packet traversing both sides - and start 
to hurt buffers

But my problem is a little different now i have no problems with RX side 
- cause i can setup anything like:

coalescence from 16 to 64

rx ring from 3000 to max 8192

And it does not change my max bw - only produces less or more interrupts.

So I start to change params for TX side - and for now i know that the 
best for me is

coalescence adaptative on

TX buffer 128

This helps with max BW that for now is close to 70Gbit/s RX and 70Gbit 
TX but after this change i have increasing DROPS on TX side for vlan 
interfaces.

And only 50% cpu (max was 50% for 70Gbit/s)


> 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.

Yes - in normal life traffic - most of ddos'es are like this many pps 
with small frames.



> - Alex
>