Messages in this thread | | | Date | Fri, 28 Jul 2023 15:50:42 +0200 | Subject | Re: [PATCH net-next 9/9] net: skbuff: always try to recycle PP pages directly when in softirq | From | Alexander Lobakin <> |
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From: Jesper Dangaard Brouer <jbrouer@redhat.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@kernel.org> # in_softirq() >> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.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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