From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 3072615748C for ; Fri, 7 Jun 2024 10:57:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1717757876; cv=none; b=Joeann91ohWaB23CbmlJS2rMuqzHfdlz79THx0tZg7ovP/pC6HsFxl6fr4mBe9AudgBLNINbpCXYeEn7IEZvNtyStOF/apbavg36FbmVzUJ/Owy072ZwgCiWc5EPC0g/e6CYAkLmb50IvBn00I0p5Msv/UfitmGFQVmmdB9Oxtk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1717757876; c=relaxed/simple; bh=6TUUxsJL4s+r/TKwLWJXF8dqzSDy/1wGhpnvu/WKfWQ=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=Qgf+Z6p4LLzzZG2HX1XlXquuugooaDni6D4diV6viHXk/ko9tGQZOBS35O7YyjIIVoQZm6Dzql9c5irnM6ZxPMCQZNTxyTSmmZGCvYftIDtaxYp38WO5IKKzNZb6dgG5f2xjAqQBxUjNBHWwZN8DD6ZT1Sa/5wLbQpeMlzG0Pdw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 002C22F4; Fri, 7 Jun 2024 03:58:19 -0700 (PDT) Received: from [10.57.70.246] (unknown [10.57.70.246]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7FACD3F762; Fri, 7 Jun 2024 03:57:53 -0700 (PDT) Message-ID: <7553070e-630e-4e86-b64e-66cfce1ee125@arm.com> Date: Fri, 7 Jun 2024 11:57:52 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm: swap: mTHP allocate swap entries from nonfull list Content-Language: en-GB From: Ryan Roberts To: Chris Li , Andrew Morton Cc: Kairui Song , "Huang, Ying" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Barry Song References: <20240524-swap-allocator-v1-0-47861b423b26@kernel.org> <20240524-swap-allocator-v1-2-47861b423b26@kernel.org> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 07/06/2024 11:35, Ryan Roberts wrote: > On 24/05/2024 18:17, Chris Li wrote: >> Track the nonfull cluster as well as the empty cluster >> on lists. Each order has one nonfull cluster list. >> >> The cluster will remember which order it was used during >> new cluster allocation. >> >> When the cluster has free entry, add to the nonfull[order] >> list.  When the free cluster list is empty, also allocate >> from the nonempty list of that order. >> >> This improves the mTHP swap allocation success rate. > > If I've understood correctly, the aim here is to link all the current per-cpu > clusters for a given order together so that if a cpu can't allocate a new > cluster for a given order, then it can steal another CPU's current cluster for > that order? > > If that's the intent, couldn't that be done just by iterating over the per-cpu, > per-order cluster pointers? Then you don't need all the linked list churn > (althogh I like the linked list changes as a nice cleanup, I'm not sure the > churn is neccessary for this change?). There would likely need to be some > locking considerations, but it would also allow you to get access to the next > entry within the cluster for allocation. > > However, fundamentally, I don't think this change solves the problem; it just > takes a bit longer before the allocation fails. The real problem is > fragmentation due to freeing individual pages from swap entries at different times. > > Wouldn't it be better to just extend scanning to support high order allocations? > Then we can steal a high order block from any cluster, even clusters that were > previously full, just like we currently do for order-0. Given we are already > falling back to this path for order-0, I don't think it would be any more > expensive; infact its less expensive because we only scan once for the high > order block, rather than scan for every split order-0 page. > > Of course that still doesn't solve the proplem entirely; if swap is so > fragmented that there is no contiguous block of the required order then you > still have to fall back to splitting. As an extra optimization, you could store > the largest contiguous free space available in each cluster to avoid scanning in > case its too small? > > >> >> There are limitations if the distribution of numbers of >> different orders of mTHP changes a lot. e.g. there are a lot >> of nonfull cluster assign to order A while later time there >> are a lot of order B allocation while very little allocation >> in order A. Currently the cluster used by order A will not >> reused by order B unless the cluster is 100% empty. >> >> This situation is best addressed by the longer term "swap >> buddy allocator", in future patches. >> --- >> include/linux/swap.h | 4 ++++ >> mm/swapfile.c | 25 +++++++++++++++++++++++-- >> 2 files changed, 27 insertions(+), 2 deletions(-) >> >> diff --git a/include/linux/swap.h b/include/linux/swap.h >> index 0d3906eff3c9..1b7f0794b9bf 100644 >> --- a/include/linux/swap.h >> +++ b/include/linux/swap.h >> @@ -255,10 +255,12 @@ struct swap_cluster_info { >> * cluster >> */ >> unsigned int count:16; >> + unsigned int order:8; >> unsigned int flags:8; >> struct list_head next; >> }; >> #define CLUSTER_FLAG_FREE 1 /* This cluster is free */ >> +#define CLUSTER_FLAG_NONFULL 2 /* This cluster is on nonfull list */ >> >> >> /* >> @@ -297,6 +299,8 @@ struct swap_info_struct { >> unsigned char *swap_map; /* vmalloc'ed array of usage counts */ >> struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */ >> struct list_head free_clusters; /* free clusters list */ >> + struct list_head nonfull_clusters[SWAP_NR_ORDERS]; >> + /* list of cluster that contains at least one free slot */ >> unsigned int lowest_bit; /* index of first free in swap_map */ >> unsigned int highest_bit; /* index of last free in swap_map */ >> unsigned int pages; /* total of usable pages of swap */ >> diff --git a/mm/swapfile.c b/mm/swapfile.c >> index 205a60c5f9cb..51923aba500e 100644 >> --- a/mm/swapfile.c >> +++ b/mm/swapfile.c >> @@ -363,8 +363,11 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si, >> >> static void __free_cluster(struct swap_info_struct *si, struct swap_cluster_info *ci) >> { >> + if (ci->flags & CLUSTER_FLAG_NONFULL) >> + list_move_tail(&ci->next, &si->free_clusters); >> + else >> + list_add_tail(&ci->next, &si->free_clusters); >> ci->flags = CLUSTER_FLAG_FREE; >> - list_add_tail(&ci->next, &si->free_clusters); >> } >> >> /* >> @@ -486,7 +489,12 @@ static void dec_cluster_info_page(struct swap_info_struct *p, struct swap_cluste >> ci->count--; >> >> if (!ci->count) >> - free_cluster(p, ci); >> + return free_cluster(p, ci); >> + >> + if (!(ci->flags & CLUSTER_FLAG_NONFULL)) { >> + list_add_tail(&ci->next, &p->nonfull_clusters[ci->order]); >> + ci->flags |= CLUSTER_FLAG_NONFULL; >> + } >> } >> >> /* >> @@ -547,6 +555,14 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si, >> ci = list_first_entry(&si->free_clusters, struct swap_cluster_info, next); >> list_del(&ci->next); >> spin_lock(&ci->lock); >> + ci->order = order; >> + ci->flags = 0; >> + spin_unlock(&ci->lock); >> + tmp = (ci - si->cluster_info) * SWAPFILE_CLUSTER; >> + } else if (!list_empty(&si->nonfull_clusters[order])) { >> + ci = list_first_entry(&si->nonfull_clusters[order], struct swap_cluster_info, next); >> + list_del(&ci->next); >> + spin_lock(&ci->lock); >> ci->flags = 0; >> spin_unlock(&ci->lock); >> tmp = (ci - si->cluster_info) * SWAPFILE_CLUSTER; > > This looks wrong to me; if the cluster is on the nonfull list then it will have > had some entries already allocated (by another cpu). So pointing tmp to the > first block in the cluster will never yield a free block. The cpu from which you > are stealing the cluster stores the next free block location in its per-cpu > structure. So perhaps iterating over the other cpu's `struct percpu_cluster`s is > a better approach than the nonfull list? Ahh; of course the cluster scan below will move this along to a free block. > > Additionally, this cluster will be stored back to this cpu's current cluster at > the bottom of the function. That may or may not be what you intended. > >> @@ -578,6 +594,7 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si, >> break; >> tmp += nr_pages; >> } >> + WARN_ONCE(ci->order != order, "expecting order %d got %d", order, ci->order); >> unlock_cluster(ci); >> } >> if (tmp >= max) { >> @@ -956,6 +973,7 @@ static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx) >> ci = lock_cluster(si, offset); >> memset(si->swap_map + offset, 0, SWAPFILE_CLUSTER); >> ci->count = 0; >> + ci->order = 0; >> ci->flags = 0; >> free_cluster(si, ci); >> unlock_cluster(ci); >> @@ -2882,6 +2900,9 @@ static int setup_swap_map_and_extents(struct swap_info_struct *p, >> INIT_LIST_HEAD(&p->free_clusters); >> INIT_LIST_HEAD(&p->discard_clusters); >> >> + for (i = 0; i < SWAP_NR_ORDERS; i++) >> + INIT_LIST_HEAD(&p->nonfull_clusters[i]); >> + >> for (i = 0; i < swap_header->info.nr_badpages; i++) { >> unsigned int page_nr = swap_header->info.badpages[i]; >> if (page_nr == 0 || page_nr > swap_header->info.last_page) >> >