Re: [PATCH net-next 9/9] net: skbuff: always try to recycle PP pages directly when in softirq

[Alexander Lobakin]

From: Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx>
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@xxxxxxxxxx> # in_softirq()
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
>> ---
>>   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