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Sat, 28 Mar 2020 00:59:32 +0000 (UTC) Received: from hqpgpgate101.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate25.nvidia.com (using TLS: TLSv1.2, DES-CBC3-SHA) id ; Fri, 27 Mar 2020 17:58:44 -0700 Received: from hqmail.nvidia.com ([172.20.161.6]) by hqpgpgate101.nvidia.com (PGP Universal service); Fri, 27 Mar 2020 17:59:31 -0700 X-PGP-Universal: processed; by hqpgpgate101.nvidia.com on Fri, 27 Mar 2020 17:59:31 -0700 Received: from [10.2.58.50] (10.124.1.5) by HQMAIL107.nvidia.com (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Sat, 28 Mar 2020 00:59:31 +0000 Subject: Re: [Patch v2 2/2] mm/page_alloc.c: define node_order with all zero To: Wei Yang CC: , , , , References: <20200327220121.27823-1-richard.weiyang@gmail.com> <20200327220121.27823-2-richard.weiyang@gmail.com> <4c9d8138-d379-810f-64e7-0d018ed019df@nvidia.com> <20200328002616.kjtf7dpkqbugyzi2@master> X-Nvconfidentiality: public From: John Hubbard Message-ID: <97a6bf40-792b-6216-d35b-691027c85aad@nvidia.com> Date: Fri, 27 Mar 2020 17:59:30 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200328002616.kjtf7dpkqbugyzi2@master> X-Originating-IP: [10.124.1.5] X-ClientProxiedBy: HQMAIL105.nvidia.com (172.20.187.12) To HQMAIL107.nvidia.com (172.20.187.13) Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1; t=1585357124; bh=INqxeXB2KvcAJdTq0p9g/PfZMMiN/uXlz75p4EJ7hPs=; h=X-PGP-Universal:Subject:To:CC:References:X-Nvconfidentiality:From: Message-ID:Date:User-Agent:MIME-Version:In-Reply-To: X-Originating-IP:X-ClientProxiedBy:Content-Type:Content-Language: Content-Transfer-Encoding; b=nJsCH9lJyqVwPMxvMHCouIMuBEfMLTgEJeYW985LA03p5ko//MtAVptCTRQ8j2Lzd s7gdUdfzfQrKXxva4KUKj2c5qI0VCIGbezMXpNRSp+M+ulwkMWxWjErbKTt2pWjsdV L9lrXOJh8iiPFwS+njQbv77frNJmBYlUe//2yBJeI0ogZMfhqWK9O8bQqSxDnZBgm8 kOi9+CXUTp4c2tAfBic2Q0bzjraYuD3O3VO9XR8OC+yGhOs+HKhSK5mP4W5JfNgnuZ 9NL2/5XCLDMLUUz/XdyqPE2g67HmgfKHGWj2lfuC9nsZNzjfe0MGAX6T2hmvsjCVRL bmTHuyL5GuPjw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 3/27/20 5:26 PM, Wei Yang wrote: > On Fri, Mar 27, 2020 at 03:37:57PM -0700, John Hubbard wrote: >> On 3/27/20 3:01 PM, Wei Yang wrote: >>> Since we always clear node_order before getting it, we can leverage >>> compiler to do this instead of at run time. >>> >>> Signed-off-by: Wei Yang >>> --- >>> mm/page_alloc.c | 3 +-- >>> 1 file changed, 1 insertion(+), 2 deletions(-) >>> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>> index dfcf2682ed40..49dd1f25c000 100644 >>> --- a/mm/page_alloc.c >>> +++ b/mm/page_alloc.c >>> @@ -5585,7 +5585,7 @@ static void build_thisnode_zonelists(pg_data_t *pgdat) >>> static void build_zonelists(pg_data_t *pgdat) >>> { >>> - static int node_order[MAX_NUMNODES]; >>> + static int node_order[MAX_NUMNODES] = {0}; >> >> >> Looks wrong: now the single instance of node_order is initialized just once by >> the compiler. And that means that only the first caller of this function >> gets a zeroed node_order array... >> > > What a shame on me. You are right, I miss the static word. > > Well, then I am curious about why we want to define it as static. Each time we > call this function, node_order would be set to 0 and find_next_best_node() > would sort a proper value into it. I don't see the reason to reserve it in a > global area and be used next time. It's not just about preserving the value. Sometimes it's about stack space. Here's the trade-offs for static variables within a function: Advantages of static variables within a function (compared to non-static variables, also within a function): ----------------------------------- * Doesn't use any of the scarce kernel stack space * Preserves values (not always necessarily and advantage) Disadvantages: ----------------------------------- * Removes basic thread safety: multiple threads can no longer independently call the function without getting interaction, and generally that means data corruption. So here, I suspect that the original motivation was probably to conserve stack space, and the author likely observed that there was no concurrency to worry about: the function was only being called by one thread at a time. Given those constraints (which I haven't confirmed just yet, btw), a static function variable fits well. > > My suggestion is to remove the static and define it {0} instead of memset > every time. Is my understanding correct here? Not completely: a) First of all, "instead of memset every time" is a misconception, because there is still a memset happening every time with {0}. It's just that the compiler silently writes that code for you, and you don't see it on the screen. But it's still there. b) Switching away from a static to an on-stack variable requires that you first verify that stack space is not an issue. Or, if you determine that this function needs the per-thread isolation that a non-static variable provides, then you can switch to either an on-stack variable, or a *alloc() function. thanks, -- John Hubbard NVIDIA