From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AC33DEAD2; Mon, 8 Jan 2024 08:59:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Received: from mail.maildlp.com (unknown [172.19.88.194]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4T7p0263HfzNkdv; Mon, 8 Jan 2024 16:59:14 +0800 (CST) Received: from dggpemm500005.china.huawei.com (unknown [7.185.36.74]) by mail.maildlp.com (Postfix) with ESMTPS id 5B9121402C6; Mon, 8 Jan 2024 16:59:50 +0800 (CST) 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_GCM_SHA256) id 15.1.2507.35; Mon, 8 Jan 2024 16:59:50 +0800 Subject: Re: [PATCH net-next 3/6] mm/page_alloc: use initial zero offset for page_frag_alloc_align() To: Alexander H Duyck , , , CC: , , Andrew Morton , References: <20240103095650.25769-1-linyunsheng@huawei.com> <20240103095650.25769-4-linyunsheng@huawei.com> From: Yunsheng Lin Message-ID: <74c9a3a1-5204-f79a-95ff-5c108ec6cf2a@huawei.com> Date: Mon, 8 Jan 2024 16:59:49 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggpemm500005.china.huawei.com (7.185.36.74) On 2024/1/5 23:42, Alexander H Duyck wrote: > On Wed, 2024-01-03 at 17:56 +0800, Yunsheng Lin wrote: >> The next patch is above to use page_frag_alloc_align() to >> replace vhost_net_page_frag_refill(), the main difference >> between those two frag page implementations is whether we >> use a initial zero offset or not. >> >> It seems more nature to use a initial zero offset, as it >> may enable more correct cache prefetching and skb frag >> coalescing in the networking, so change it to use initial >> zero offset. >> >> Signed-off-by: Yunsheng Lin >> CC: Alexander Duyck > > There are several advantages to running the offset as a countdown > rather than count-up value. > > 1. Specifically for the size of the chunks we are allocating doing it > from the bottom up doesn't add any value as we are jumping in large > enough amounts and are being used for DMA so being sequential doesn't > add any value. What is the expected size of the above chunks in your mind? I suppose that is like NAPI_HAS_SMALL_PAGE_FRAG to avoid excessive truesize underestimation? It seems there is no limit for min size of allocation for page_frag_alloc_align() now, and as the page_frag API seems to be only used in networking, should we enforce the min size of allocation at API level? > > 2. By starting at the end and working toward zero we can use built in > functionality of the CPU to only have to check and see if our result > would be signed rather than having to load two registers with the > values and then compare them which saves us a few cycles. In addition > it saves us from having to read both the size and the offset for every > page. I suppose the above is ok if we only use the page_frag_alloc*() API to allocate memory for skb->data, not for the frag in skb_shinfo(), as by starting at the end and working toward zero, it means we can not do skb coalescing. As page_frag_alloc*() is returning va now, I am assuming most of users is using the API for skb->data, I guess it is ok to drop this patch for now. If we allow page_frag_alloc*() to return struct page, we might need this patch to enable coalescing. > > Again this is another code cleanup at the cost of performance. I > realize many of the items you are removing would be considered micro- > optimizations but when we are dealing with millions of packets per > second those optimizations add up. > . >