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[94.66.220.137]) by smtp.gmail.com with ESMTPSA id l10sm3237715wrg.50.2021.09.16.03.38.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Sep 2021 03:38:15 -0700 (PDT) Date: Thu, 16 Sep 2021 13:38:12 +0300 From: Ilias Apalodimas To: Yunsheng Lin Cc: Jesper Dangaard Brouer , brouer@redhat.com, Alexander Duyck , davem@davemloft.net, kuba@kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linuxarm@openeuler.org, hawk@kernel.org, jonathan.lemon@gmail.com, alobakin@pm.me, willemb@google.com, cong.wang@bytedance.com, pabeni@redhat.com, haokexin@gmail.com, nogikh@google.com, elver@google.com, memxor@gmail.com, edumazet@google.com, dsahern@gmail.com Subject: Re: [Linuxarm] Re: [PATCH net-next v2 3/3] skbuff: keep track of pp page when __skb_frag_ref() is called Message-ID: References: <20210914121114.28559-1-linyunsheng@huawei.com> <20210914121114.28559-4-linyunsheng@huawei.com> <9467ec14-af34-bba4-1ece-6f5ea199ec97@huawei.com> <0337e2f6-5428-2c75-71a5-6db31c60650a@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 16, 2021 at 05:33:39PM +0800, Yunsheng Lin wrote: > On 2021/9/16 16:44, Ilias Apalodimas wrote: > >>>> appear if we try to pull in your patches on using page pool and recycling > > > > [...] > > > >>>> for Tx where TSO and skb_split are used? > >> > >> As my understanding, the problem might exists without tx recycling, because a > >> skb from wire would be passed down to the tcp stack and retransmited back to > >> the wire theoretically. As I am not able to setup a configuration to verify > >> and test it and the handling seems tricky, so I am targetting net-next branch > >> instead of net branch. > >> > >>>> > >>>> I'll be honest, when I came up with the recycling idea for page pool, I > >>>> never intended to support Tx. I agree with Alexander here, If people want > >>>> to use it on Tx and think there's value, we might need to go back to the > >>>> drawing board and see what I've missed. It's still early and there's a > >>>> handful of drivers using it, so it will less painful now. > >> > >> Yes, we also need to prototype it to see if there is something missing in the > >> drawing board and how much improvement we get from that:) > >> > >>> > >>> I agree, page_pool is NOT designed or intended for TX support. > >>> E.g. it doesn't make sense to allocate a page_pool instance per socket, as the backing memory structures for page_pool are too much. > >>> As the number RX-queues are more limited it was deemed okay that we use page_pool per RX-queue, which sacrifice some memory to gain speed. > >> > >> As memtioned before, Tx recycling is based on page_pool instance per socket. > >> it shares the page_pool instance with rx. > >> > >> Anyway, based on feedback from edumazet and dsahern, I am still trying to > >> see if the page pool is meaningful for tx. > >> > >>> > >>> > >>>> The pp_recycle_bit was introduced to make the checking faster, instead of > >>>> getting stuff into cache and check the page signature. If that ends up > >>>> being counterproductive, we could just replace the entire logic with the > >>>> frag count and the page signature, couldn't we? In that case we should be > >>>> very cautious and measure potential regression on the standard path. > >>> > >>> +1 > >> > >> I am not sure "pp_recycle_bit was introduced to make the checking faster" is a > >> valid. The size of "struct page" is only about 9 words(36/72 bytes), which is > >> mostly to be in the same cache line, and both standard path and recycle path have > >> been touching the "struct page", so it seems the overhead for checking signature > >> seems minimal. > >> > >> I agree that we need to be cautious and measure potential regression on the > >> standard path. > > > > well pp_recycle is on the same cache line boundary with the head_frag we > > need to decide on recycling. After that we start checking page signatures > > etc, which means the default release path remains mostly unaffected. > > > > I guess what you are saying here, is that 'struct page' is going to be > > accessed eventually by the default network path, so there won't be any > > noticeable performance hit? What about the other usecases we have > > Yes. In that case you'd need to call virt_to_head_page() early though, get it and then compare the signature. I guess that's avoidable by using frag->bv_page for the fragments? > > > for pp_recycle right now? __skb_frag_unref() in skb_shift() or > > skb_try_coalesce() (the latter can probably be removed tbh). > > If we decide to go with accurate indicator of a pp page, we just need > to make sure network stack use __skb_frag_unref() and __skb_frag_ref() > to put and get a page frag, the indicator checking need only done in > __skb_frag_unref() and __skb_frag_ref(), so the skb_shift() and > skb_try_coalesce() should be fine too. > > > > >> > >> Another way is to use the bit 0 of frag->bv_page ptr to indicate if a frag > >> page is from page pool. > > > > Instead of the 'struct page' signature? And the pp_recycle bit will > > continue to exist? > > pp_recycle bit might only exist or is only used for the head page for the skb. > The bit 0 of frag->bv_page ptr can be used to indicate a frag page uniquely. > Doing a memcpying of shinfo or "*fragto = *fragfrom" automatically pass the > indicator to the new shinfo before doing a __skb_frag_ref(), and __skb_frag_ref() > will increment the _refcount or pp_frag_count according to the bit 0 of > frag->bv_page. > > By the way, I also prototype the above idea, and it seems to work well too. > As long as no one else touches this, it's just another way of identifying a page_pool allocated page. But are we gaining by that? Not using virt_to_head_page() as stated above? But in that case you still need to keep pp_recycle around. > > . > > Right now the 'naive' explanation on the recycling decision is something like: > > > > if (pp_recycle) <--- recycling bit is set > > (check page signature) <--- signature matches page pool > > (check fragment refcnt) <--- If frags are enabled and is the last consumer > > recycle > > > > If we can proove the performance is unaffected when we eliminate the first if, > > then obviously we should remove it. I'll try running that test here and see, > > but keep in mind I am only testing on an 1GB interface. Any chance we can get > > measurements on a beefier hardware using hns3 ? > > Sure, I will try it. > As the kind of performance overhead is small, any performance testcase in mind? > 'eliminate the first if' wasn't accurate. I meant switch the first if and check the struct page signature instead. That would be the best solution imho. We effectively have a single rule to check if a packet comes from page_pool or not. You can start by sending a lot of packets and dropping those immediately. That should put enough stress on the receive path and the allocators and it should give us a rough idea. > > > >> > >>> > >>>> But in general, I'd be happier if we only had a simple logic in our > >>>> testing for the pages we have to recycle. Debugging and understanding this > >>>> otherwise will end up being a mess. > >>> > >>> > > > > [...] > > > > Regards > > /Ilias > > . > > Regards /Ilias