On 03/11/2018 2:53 PM, Jesper Dangaard Brouer wrote:
>
> On Fri, 2 Nov 2018 22:20:24 +0800 Aaron Lu <aaron...@intel.com> wrote:
>
>> On Fri, Nov 02, 2018 at 12:40:37PM +0100, Jesper Dangaard Brouer wrote:
>>> On Fri, 2 Nov 2018 13:23:56 +0800
>>> Aaron Lu <aaron...@intel.com> wrote:
>>>
>>>> On Thu, Nov 01, 2018 at 08:23:19PM +0000, Saeed Mahameed wrote:
>>>>> On Thu, 2018-11-01 at 23:27 +0800, Aaron Lu wrote:
>>>>>> On Thu, Nov 01, 2018 at 10:22:13AM +0100, Jesper Dangaard Brouer
>>>>>> wrote:
>>>>>> ... ...
>>>>>>> Section copied out:
>>>>>>>
>>>>>>> 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
>>>>>>
>>>>>> This callchain looks like it is freeing higher order pages than order
>>>>>> 0:
>>>>>> __free_pages_ok is only called for pages whose order are bigger than
>>>>>> 0.
>>>>>
>>>>> mlx5 rx uses only order 0 pages, so i don't know where these high order
>>>>> tx SKBs are coming from..
>>>>
>>>> Perhaps here:
>>>> __netdev_alloc_skb(), __napi_alloc_skb(), __netdev_alloc_frag() and
>>>> __napi_alloc_frag() will all call page_frag_alloc(), which will use
>>>> __page_frag_cache_refill() to get an order 3 page if possible, or fall
>>>> back to an order 0 page if order 3 page is not available.
>>>>
>>>> I'm not sure if your workload will use the above code path though.
>>>
>>> TL;DR: this is order-0 pages (code-walk trough proof below)
>>>
>>> To Aaron, the network stack *can* call __free_pages_ok() with order-0
>>> pages, via:
>>>
>>> static void skb_free_head(struct sk_buff *skb)
>>> {
>>> unsigned char *head = skb->head;
>>>
>>> if (skb->head_frag)
>>> skb_free_frag(head);
>>> else
>>> kfree(head);
>>> }
>>>
>>> static inline void skb_free_frag(void *addr)
>>> {
>>> page_frag_free(addr);
>>> }
>>>
>>> /*
>>> * Frees a page fragment allocated out of either a compound or order 0
>>> page.
>>> */
>>> 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));
>>> }
>>> EXPORT_SYMBOL(page_frag_free);
>>
>> I think here is a problem - order 0 pages are freed directly to buddy,
>> bypassing per-cpu-pages. This might be the reason lock contention
>> appeared on free path.
>
> OMG - you just found a significant issue with the network stacks
> interaction with the page allocator! This explains why I could not get
> the PCP (Per-Cpu-Pages) system to have good performance, in my
> performance networking benchmarks. As we are basically only using the
> alloc side of PCP, and not the free side.
> We have spend years adding different driver level recycle tricks to
> avoid this code path getting activated, exactly because it is rather
> slow and problematic that we hit this zone->lock.
>
Oh! It has been behaving this way for too long.
Good catch!
>> Can someone apply below diff and see if lock contention is gone?
>
> I have also applied and tested this patch, and yes the lock contention
> is gone. As mentioned is it rather difficult to hit this code path, as
> the driver page recycle mechanism tries to hide/avoid it, but mlx5 +
> page_pool + CPU-map recycling have a known weakness that bypass the
> driver page recycle scheme (that I've not fixed yet). I observed a 7%
> speedup for this micro benchmark.
>
Great news. I also have a benchmark that uses orde-r0 pages and stresses
the zone-lock. I'll test your patch during this week.
>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index e2ef1c17942f..65c0ae13215a 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4554,8 +4554,14 @@ 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);
>
> Thank you Aaron for spotting this!!!
>
Thanks Aaron :) !!
Does it conflict with your recent work that optimizes order-0 allocation?
