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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AD101C6FD18 for ; Wed, 29 Mar 2023 16:23:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229849AbjC2QXP (ORCPT ); Wed, 29 Mar 2023 12:23:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46420 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229910AbjC2QXK (ORCPT ); Wed, 29 Mar 2023 12:23:10 -0400 Received: from mail-wm1-x32b.google.com (mail-wm1-x32b.google.com [IPv6:2a00:1450:4864:20::32b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8DCA0619C for ; Wed, 29 Mar 2023 09:23:08 -0700 (PDT) Received: by mail-wm1-x32b.google.com with SMTP id v20-20020a05600c471400b003ed8826253aso2362019wmo.0 for ; Wed, 29 Mar 2023 09:23:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; t=1680106987; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=GlibarqBbZ4Q0mPsWReLWgLf3BnPWtPZlsUlpY0M8PQ=; b=RSO+my1D55gcMaKHDx5zQ3TCPd3DaIBAgXOqlGdI8tctkJGeKTNSV8irKwETa6Qr/q ygvt0qW05COhtQdgjp79ou02U9SeEA05pUU047PBx83ycFUMVKrQHgKZWDZWeRgeV+3/ ZFagReelZMGVg+QmwFTVVu+UMiiQ1OQSLo8DV73K0Kl2GPv6aKVD/lGZ3dk2Vdmgt8ww rH7lnBjcZuNzajuALBDctf7jOSp84d6TTAYW6pM1Fxyw6CL3Fc1ov3CVNpYtYQ80PBFZ RudhmGJxext9IcHG3VmYjZ4Xz+B5XDLLvnBJ2EIUGJaogZjnhRKGcAajlA6iJ0yVYfYr 9vjg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; t=1680106987; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=GlibarqBbZ4Q0mPsWReLWgLf3BnPWtPZlsUlpY0M8PQ=; b=ec0K4J6YNdxk3zQG0YM1JSfo1PYvUtcREYmjo6L2NNWF48RZjRqC0clRwFXDHTFzIR Savwcg9at4ONmGJbNp7WuVjavkA/IAD2GF8df1ciG/auS7z3bmvFVEAHQy+Ma/NDoD52 kVj1WoBvHMq3bFqOk+/caLL2TOW+gNl7GiuD1e0zrFT/xIvHAJFxgb1U5SkMEflZBeJC idgs9vDFKgdEFBcSfoDMNDRBUo/+HPIe2Jy3hZg3FBF2zwb72nln2n1ECktLtv4nyFAd drGW+hoz/Sks2bDicET30nylPwmzIou/B89MZoxsJ5f+dqpcGgpc/LEH7fpIlvZGdRmF zD/g== X-Gm-Message-State: AO0yUKUIJtBBmj7Vt0ZCsdlCelFejauuO8rqjAYgswh7FaZUY6frN3Mc ez0cMPR8p7Hxih4G+q7J+XNKab01Rac= X-Google-Smtp-Source: AK7set+kPI0uPHX/+oeAOe4CloIOapYtWqi+nps/c/zQdsmvNuNSHHfB15M0OOakmiPQu34zcSwxtw== X-Received: by 2002:a1c:790b:0:b0:3ee:56f7:75d2 with SMTP id l11-20020a1c790b000000b003ee56f775d2mr15645326wme.20.1680106986790; Wed, 29 Mar 2023 09:23:06 -0700 (PDT) Received: from localhost ([208.34.186.1]) by smtp.gmail.com with ESMTPSA id x15-20020a05600c21cf00b003ee1b2ab9a0sm2699424wmj.11.2023.03.29.09.23.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Mar 2023 09:23:05 -0700 (PDT) Date: Wed, 29 Mar 2023 17:23:04 +0100 From: Lorenzo Stoakes To: Uladzislau Rezki Cc: Andrew Morton , linux-mm@kvack.org, LKML , Baoquan He , Christoph Hellwig , Matthew Wilcox , Dave Chinner , Oleksiy Avramchenko Subject: Re: [PATCH v3 1/2] mm: vmalloc: Remove a global vmap_blocks xarray Message-ID: References: <20230327170126.406044-1-urezki@gmail.com> <132e2d5c-0c1f-4fff-850c-b3fb084455bb@lucifer.local> 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 Wed, Mar 29, 2023 at 05:01:11PM +0200, Uladzislau Rezki wrote: > Hello, Lorenzo! > > > > /* > > > - * XArray of vmap blocks, indexed by address, to quickly find a vmap block > > > - * in the free path. Could get rid of this if we change the API to return a > > > - * "cookie" from alloc, to be passed to free. But no big deal yet. > > > + * In order to fast access to any "vmap_block" associated with a > > > + * specific address, we store them into a per-cpu xarray. A hash > > > + * function is addr_to_vbq() whereas a key is a vb->va->va_start > > > + * value. > > > + * > > > + * Please note, a vmap_block_queue, which is a per-cpu, is not > > > + * serialized by a raw_smp_processor_id() current CPU, instead > > > + * it is chosen based on a CPU-index it belongs to, i.e. it is > > > + * a hash-table. > > > + * > > > + * An example: > > > + * > > > + * CPU_1 CPU_2 CPU_0 > > > + * | | | > > > + * V V V > > > + * 0 10 20 30 40 50 60 > > > + * |------|------|------|------|------|------|... > > > + * CPU0 CPU1 CPU2 CPU0 CPU1 CPU2 > > > + * > > > + * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus > > > + * it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock; > > > + * > > > + * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus > > > + * it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock; > > > + * > > > + * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus > > > + * it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock. > > > */ > > > > OK so if I understand this correctly, you're overloading the per-CPU > > vmap_block_queue array to use as a simple hash based on the address and > > relying on the xa_lock() in xa_insert() to serialise in case of contention? > > > > I like the general heft of your comment but I feel this could be spelled > > out a little more clearly, something like:- > > > > In order to have fast access to any vmap_block object associated with a > > specific address, we use a hash. > > > > Rather than waste space on defining a new hash table we take advantage > > of the fact we already have a static per-cpu array vmap_block_queue. > > > > This is already used for per-CPU access to the block queue, however we > > overload this to _also_ act as a vmap_block hash. The hash function is > > addr_to_vbq() which hashes on vb->va->va_start. > > > > This then uses per_cpu() to lookup the _index_ rather than the > > _cpu_. Each vmap_block_queue contains an xarray of vmap blocks which are > > indexed on the same key as the hash (vb->va->va_start). > > > > xarray read acceses are protected by RCU lock and inserts are protected > > by a spin lock so there is no risk of a race here. > > > /* > * In order to fast access to any "vmap_block" associated with a > * specific address, we use a hash. > * > * A per-cpu vmap_block_queue is used in both ways, to serialize > * an access to free block chains among CPUs(alloc path) and it > * also acts as a vmap_block hash(alloc/free paths). It means we > * overload it, since we already have the per-cpu array which is > * used as a hash table. Nit - it may be worth highlighting that when used as a hash it the 'cpu' is not in fact a cpu but rather a hash key. E.g. just add on the end of this something like:- When used as a hash table the 'cpu' passed to per_cpu is not actually a CPU but rather the hash key. > * > * A hash function is addr_to_vbq() which hashes any address to > * a specific index(in a hash) it belongs to. This then uses a > * per_cpu() macro to access the array with specific index. May need a tweak if you are happy with my review that we can simply have a helper that returns the xarray in which case we won't necessary have this function :) but depends of course on how the respin looks! > * > * An example: > * > * CPU_1 CPU_2 CPU_0 > * | | | > * V V V > * 0 10 20 30 40 50 60 > * |------|------|------|------|------|------|... > * CPU0 CPU1 CPU2 CPU0 CPU1 CPU2 > * > * - CPU_1 invokes vm_unmap_ram(6), 6 belongs to CPU0 zone, thus > * it access: CPU0/INDEX0 -> vmap_blocks -> xa_lock; > * > * - CPU_2 invokes vm_unmap_ram(11), 11 belongs to CPU1 zone, thus > * it access: CPU1/INDEX1 -> vmap_blocks -> xa_lock; > * > * - CPU_0 invokes vm_unmap_ram(20), 20 belongs to CPU2 zone, thus > * it access: CPU2/INDEX2 -> vmap_blocks -> xa_lock. > * > * This technique allows almost remove a lock-contention in locking > * primitives which protect insert/remove operations. This sentence is a little confusing, perhaps rephrase a little:- This technique almost always avoids lock contention on insert/remove, however the xarray spinlock protects against any contention that remains. > */ > Are you find with it? Other than the small nits above (sorry!) it seems fine! Thanks for updating, much appreciated :) > > -- > Uladzislau Rezki >