| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <08828f13-1e47-e65b-caa8-2319167fc495@itcare.pl>
Date: Fri, 2 Nov 2018 20:02:12 +0100
From: Paweł Staszewski <pstaszewski@...are.pl>
To: Aaron Lu <aaron.lu@...el.com>,
Jesper Dangaard Brouer <brouer@...hat.com>
Cc: Saeed Mahameed <saeedm@...lanox.com>,
"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
Tariq Toukan <tariqt@...lanox.com>,
"ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
"yoel@...knet.dk" <yoel@...knet.dk>,
"mgorman@...hsingularity.net" <mgorman@...hsingularity.net>
Subject: Re: Kernel 4.19 network performance - forwarding/routing normal users
traffic
W dniu 02.11.2018 o 15:20, Aaron Lu pisze:
> 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.lu@...el.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. Can someone apply below diff and see if lock
> contention is gone?
Will test it tonight
>
> 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);
>
>> Notice for the mlx5 driver it support several RX-memory models, so it
>> can be hard to follow, but from the perf report output we can see that
>> is uses mlx5e_skb_from_cqe_linear, which use build_skb.
>>
>> --13.63%--mlx5e_skb_from_cqe_linear
>> |
>> --5.02%--build_skb
>> |
>> --1.85%--__build_skb
>> |
>> --1.00%--kmem_cache_alloc
>>
>> /* build_skb() is wrapper over __build_skb(), that specifically
>> * takes care of skb->head and skb->pfmemalloc
>> * This means that if @frag_size is not zero, then @data must be backed
>> * by a page fragment, not kmalloc() or vmalloc()
>> */
>> struct sk_buff *build_skb(void *data, unsigned int frag_size)
>> {
>> struct sk_buff *skb = __build_skb(data, frag_size);
>>
>> if (skb && frag_size) {
>> skb->head_frag = 1;
>> if (page_is_pfmemalloc(virt_to_head_page(data)))
>> skb->pfmemalloc = 1;
>> }
>> return skb;
>> }
>> EXPORT_SYMBOL(build_skb);
>>
>> It still doesn't prove, that the @data is backed by by a order-0 page.
>> For the mlx5 driver is uses mlx5e_page_alloc_mapped ->
>> page_pool_dev_alloc_pages(), and I can see perf report using
>> __page_pool_alloc_pages_slow().
>>
>> The setup for page_pool in mlx5 uses order=0.
>>
>> /* Create a page_pool and register it with rxq */
>> pp_params.order = 0;
>> pp_params.flags = 0; /* No-internal DMA mapping in page_pool */
>> pp_params.pool_size = pool_size;
>> pp_params.nid = cpu_to_node(c->cpu);
>> pp_params.dev = c->pdev;
>> pp_params.dma_dir = rq->buff.map_dir;
>>
>> /* page_pool can be used even when there is no rq->xdp_prog,
>> * given page_pool does not handle DMA mapping there is no
>> * required state to clear. And page_pool gracefully handle
>> * elevated refcnt.
>> */
>> rq->page_pool = page_pool_create(&pp_params);
>> if (IS_ERR(rq->page_pool)) {
>> err = PTR_ERR(rq->page_pool);
>> rq->page_pool = NULL;
>> goto err_free;
>> }
>> err = xdp_rxq_info_reg_mem_model(&rq->xdp_rxq,
>> MEM_TYPE_PAGE_POOL, rq->page_pool);
> Thanks for the detailed analysis, I'll need more time to understand the
> whole picture :-)
>
Powered by blists - more mailing lists