From nobody Mon Feb 9 00:01:08 2026 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id ED814446A0 for ; Mon, 11 Mar 2024 15:01:17 +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=1710169279; cv=none; b=KBnsMG1h8mbfH3lXBztFtyljv9WS5EF5VDk/7iRQhDUuccvEjGY1XjRkI9oXK7CBGmOlI321hbTnmzar46uRiGcQpowXyw23aUv0mu/kPmb0tFxjG/10HmFRnWJluQVuBwIo9jVnjn1BAXnZ/UcnoehWVOisJbKSe894basV0MM= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710169279; c=relaxed/simple; bh=qN/KxcJbwbD57I3Qfyamd12HqhbZFxxTeZAyoxMTOM4=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=kbcGAaQFhDTfCE3ZB1mzofn3xEKc34zJXVLs7gfusthSw7QZX8Y47+d7XoqAde3SIBHD2SuCK4ULGgmvxraD4z87amZhN84n52cRuxScrrlTpmpKnkghvT+rPq+xpjVdGTGtnp2AGicNo8JogZ7NK/r/v7Nv1G0+snR32KOYvyE= 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 1953C1570; Mon, 11 Mar 2024 08:01:54 -0700 (PDT) Received: from e125769.cambridge.arm.com (e125769.cambridge.arm.com [10.1.196.27]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3126C3F64C; Mon, 11 Mar 2024 08:01:15 -0700 (PDT) From: Ryan Roberts To: Andrew Morton , David Hildenbrand , Matthew Wilcox , Huang Ying , Gao Xiang , Yu Zhao , Yang Shi , Michal Hocko , Kefeng Wang , Barry Song <21cnbao@gmail.com>, Chris Li Cc: Ryan Roberts , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH v4 3/6] mm: swap: Simplify struct percpu_cluster Date: Mon, 11 Mar 2024 15:00:55 +0000 Message-Id: <20240311150058.1122862-4-ryan.roberts@arm.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20240311150058.1122862-1-ryan.roberts@arm.com> References: <20240311150058.1122862-1-ryan.roberts@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" struct percpu_cluster stores the index of cpu's current cluster and the offset of the next entry that will be allocated for the cpu. These two pieces of information are redundant because the cluster index is just (offset / SWAPFILE_CLUSTER). The only reason for explicitly keeping the cluster index is because the structure used for it also has a flag to indicate "no cluster". However this data structure also contains a spin lock, which is never used in this context, as a side effect the code copies the spinlock_t structure, which is questionable coding practice in my view. So let's clean this up and store only the next offset, and use a sentinal value (SWAP_NEXT_INVALID) to indicate "no cluster". SWAP_NEXT_INVALID is chosen to be 0, because 0 will never be seen legitimately; The first page in the swap file is the swap header, which is always marked bad to prevent it from being allocated as an entry. This also prevents the cluster to which it belongs being marked free, so it will never appear on the free list. This change saves 16 bytes per cpu. And given we are shortly going to extend this mechanism to be per-cpu-AND-per-order, we will end up saving 16 * 9 =3D 144 bytes per cpu, which adds up if you have 256 cpus in the system. Signed-off-by: Ryan Roberts Reviewed-by: "Huang, Ying" --- include/linux/swap.h | 9 ++++++++- mm/swapfile.c | 22 +++++++++++----------- 2 files changed, 19 insertions(+), 12 deletions(-) diff --git a/include/linux/swap.h b/include/linux/swap.h index f2b7f204b968..0cb082bee717 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -260,13 +260,20 @@ struct swap_cluster_info { #define CLUSTER_FLAG_FREE 1 /* This cluster is free */ #define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */ =20 +/* + * The first page in the swap file is the swap header, which is always mar= ked + * bad to prevent it from being allocated as an entry. This also prevents = the + * cluster to which it belongs being marked free. Therefore 0 is safe to u= se as + * a sentinel to indicate next is not valid in percpu_cluster. + */ +#define SWAP_NEXT_INVALID 0 + /* * We assign a cluster to each CPU, so each CPU can allocate swap entry fr= om * its own cluster and swapout sequentially. The purpose is to optimize sw= apout * throughput. */ struct percpu_cluster { - struct swap_cluster_info index; /* Current cluster index */ unsigned int next; /* Likely next allocation offset */ }; =20 diff --git a/mm/swapfile.c b/mm/swapfile.c index ee7e44cb40c5..3828d81aa6b8 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -609,7 +609,7 @@ scan_swap_map_ssd_cluster_conflict(struct swap_info_str= uct *si, return false; =20 percpu_cluster =3D this_cpu_ptr(si->percpu_cluster); - cluster_set_null(&percpu_cluster->index); + percpu_cluster->next =3D SWAP_NEXT_INVALID; return true; } =20 @@ -622,14 +622,14 @@ static bool scan_swap_map_try_ssd_cluster(struct swap= _info_struct *si, { struct percpu_cluster *cluster; struct swap_cluster_info *ci; - unsigned long tmp, max; + unsigned int tmp, max; =20 new_cluster: cluster =3D this_cpu_ptr(si->percpu_cluster); - if (cluster_is_null(&cluster->index)) { + tmp =3D cluster->next; + if (tmp =3D=3D SWAP_NEXT_INVALID) { if (!cluster_list_empty(&si->free_clusters)) { - cluster->index =3D si->free_clusters.head; - cluster->next =3D cluster_next(&cluster->index) * + tmp =3D cluster_next(&si->free_clusters.head) * SWAPFILE_CLUSTER; } else if (!cluster_list_empty(&si->discard_clusters)) { /* @@ -649,9 +649,7 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_i= nfo_struct *si, * Other CPUs can use our cluster if they can't find a free cluster, * check if there is still free entry in the cluster */ - tmp =3D cluster->next; - max =3D min_t(unsigned long, si->max, - (cluster_next(&cluster->index) + 1) * SWAPFILE_CLUSTER); + max =3D min_t(unsigned long, si->max, ALIGN(tmp + 1, SWAPFILE_CLUSTER)); if (tmp < max) { ci =3D lock_cluster(si, tmp); while (tmp < max) { @@ -662,12 +660,13 @@ static bool scan_swap_map_try_ssd_cluster(struct swap= _info_struct *si, unlock_cluster(ci); } if (tmp >=3D max) { - cluster_set_null(&cluster->index); + cluster->next =3D SWAP_NEXT_INVALID; goto new_cluster; } - cluster->next =3D tmp + 1; *offset =3D tmp; *scan_base =3D tmp; + tmp +=3D 1; + cluster->next =3D tmp < max ? tmp : SWAP_NEXT_INVALID; return true; } =20 @@ -3138,8 +3137,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialf= ile, int, swap_flags) } for_each_possible_cpu(cpu) { struct percpu_cluster *cluster; + cluster =3D per_cpu_ptr(p->percpu_cluster, cpu); - cluster_set_null(&cluster->index); + cluster->next =3D SWAP_NEXT_INVALID; } } else { atomic_inc(&nr_rotate_swap); --=20 2.25.1