From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B52F2C433FE for ; Thu, 16 Sep 2021 11:05:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 92F4E61242 for ; Thu, 16 Sep 2021 11:05:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237292AbhIPLG1 (ORCPT ); Thu, 16 Sep 2021 07:06:27 -0400 Received: from szxga08-in.huawei.com ([45.249.212.255]:16219 "EHLO szxga08-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235657AbhIPLGT (ORCPT ); Thu, 16 Sep 2021 07:06:19 -0400 Received: from dggemv703-chm.china.huawei.com (unknown [172.30.72.54]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4H9DjN5zVdz13VdT; Thu, 16 Sep 2021 19:03:52 +0800 (CST) Received: from dggpemm500005.china.huawei.com (7.185.36.74) by dggemv703-chm.china.huawei.com (10.3.19.46) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.8; Thu, 16 Sep 2021 19:04:54 +0800 Received: from [10.69.30.204] (10.69.30.204) by dggpemm500005.china.huawei.com (7.185.36.74) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256) id 15.1.2308.8; Thu, 16 Sep 2021 19:04:54 +0800 Subject: Re: [Linuxarm] Re: [PATCH net-next v2 3/3] skbuff: keep track of pp page when __skb_frag_ref() is called To: Ilias Apalodimas CC: Jesper Dangaard Brouer , , Alexander Duyck , , , , , , , , , , , , , , , , , 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> From: Yunsheng Lin Message-ID: Date: Thu, 16 Sep 2021 19:04:54 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.69.30.204] X-ClientProxiedBy: dggeme705-chm.china.huawei.com (10.1.199.101) To dggpemm500005.china.huawei.com (7.185.36.74) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/9/16 18:38, Ilias Apalodimas wrote: > 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? If a page of a skb frag is from page pool, It seems frag->bv_page is always point to head_page of a compound page, so the calling of virt_to_head_page() does not seems necessary. bit 0 of frag->bv_page is different way of indicatior for a pp page, it is better we do not confuse with the page signature way. Using a bit 0 may give us a free word in 'struct page' if we manage to use skb->pp_recycle to indicate a head page of the skb uniquely, meaning page->pp_magic can be used for future feature. > >> >>> 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. No, we do not need the pp_recycle, as long as the we make sure __skb_frag_ref() is called after memcpying the shinfo or doing "*fragto = *fragfrom". > >>> . >>> 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. I am not sure what does "switch " means here, if the page signature can indicate a pp page uniquely, the "if (pp_recycle)" checking can be removed. > > 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 > . >