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Message-ID: <38c06c0e-7467-2596-133f-05c5c1569ccc@intel.com>
Date: Fri, 28 Jul 2023 15:50:42 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jesper Dangaard Brouer <jbrouer@...hat.com>
CC: "David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, <brouer@...hat.com>,
Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
Larysa Zaremba <larysa.zaremba@...el.com>,
Yunsheng Lin <linyunsheng@...wei.com>,
Alexander Duyck <alexanderduyck@...com>,
Jesper Dangaard Brouer <hawk@...nel.org>,
"Ilias Apalodimas" <ilias.apalodimas@...aro.org>,
Simon Horman <simon.horman@...igine.com>,
<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<qingfang.deng@...lower.com.cn>
Subject: Re: [PATCH net-next 9/9] net: skbuff: always try to recycle PP pages
directly when in softirq
From: Jesper Dangaard Brouer <jbrouer@...hat.com>
Date: Fri, 28 Jul 2023 11:32:01 +0200
>
>
> On 27/07/2023 16.43, Alexander Lobakin wrote:
>> Commit 8c48eea3adf3 ("page_pool: allow caching from safely localized
>> NAPI") allowed direct recycling of skb pages to their PP for some cases,
>> but unfortunately missed a couple of other majors.
>> For example, %XDP_DROP in skb mode. The netstack just calls kfree_skb(),
>> which unconditionally passes `false` as @napi_safe. Thus, all pages go
>> through ptr_ring and locks, although most of time we're actually inside
>> the NAPI polling this PP is linked with, so that it would be perfectly
>> safe to recycle pages directly.
>
> The commit messages is hard to read. It would help me as the reader if
> you used a empty line between paragraphs, like in this location (same
> goes for other commit descs).
O_o
I paste empty line basing on logics. These two don't have it, as the
second paragraph is the continuation of the first: it expands what I
mean by "a couple of other majors".
Do you want to have empty newlines between each paragraph instead?
>
>> Let's address such. If @napi_safe is true, we're fine, don't change
>> anything for this path. But if it's false, check whether we are in the
>> softirq context. It will most likely be so and then if ->list_owner
>> is our current CPU, we're good to use direct recycling, even though
>> @napi_safe is false -- concurrent access is excluded. in_softirq()
>> protection is needed mostly due to we can hit this place in the
>> process context (not the hardirq though).
>
> This patch make me a little nervous, as it can create hard-to-debug bugs
> if this isn't 100% correct. (Thanks for previous patch that exclude
> hardirq via lockdep).
Pretty much any -next patch can create "hard-to-debug" bugs. Not a
reason to avoid any improvements, tho?
Speaking of this particular patch, can you give an example of situation
where this wouldn't be correct?
>
>> For the mentioned xdp-drop-skb-mode case, the improvement I got is
>> 3-4% in Mpps. As for page_pool stats, recycle_ring is now 0 and
>> alloc_slow counter doesn't change most of time, which means the
>> MM layer is not even called to allocate any new pages.
>>
>> Suggested-by: Jakub Kicinski <kuba@...nel.org> # in_softirq()
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
>> ---
>> net/core/skbuff.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> index e701401092d7..5ba3948cceed 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -901,8 +901,10 @@ bool page_pool_return_skb_page(struct page *page,
>> bool napi_safe)
>> /* Allow direct recycle if we have reasons to believe that we are
>> * in the same context as the consumer would run, so there's
>> * no possible race.
>> + * __page_pool_put_page() makes sure we're not in hardirq context
>> + * and interrupts are enabled prior to accessing the cache.
>> */
>> - if (napi_safe) {
>> + if (napi_safe || in_softirq()) {
>
> I used to have in_serving_softirq() in PP to exclude process context
> that just disabled BH to do direct recycling (into a lockless array).
> This changed in kernel v6.3 commit 542bcea4be86 ("net: page_pool: use
> in_softirq() instead") to help threaded NAPI. I guess, nothing blew up
> so I guess this was okay to relax this.
(below)
>
>> const struct napi_struct *napi = READ_ONCE(pp->p.napi);
>> allow_direct = napi &&
>
> AFAIK this in_softirq() will allow process context with disabled BH to
> also recycle directly into the PP lockless array. With the additional
> checks (that are just outside above diff-context) that I assume makes
> sure CPU (smp_processor_id()) also match. Is this safe?
Disabling BH also disables preemption. smp_processor_id() can give wrong
values only when preemption is enabled (see get_cpu()/put_cpu()).
Also look at how threaded NAPI and busy polling call NAPI polling
callbacks. They just disable BH. And nobody ever said that it's not safe
to call smp_processor_id() in the NAPI polling callbacks.
When your context matches and the processor ID matches, how could you
provoke concurrent access?
>
> --Jesper
>
Thanks,
Olek
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