From patchwork Thu Jun 2 18:54:01 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Waiman Long X-Patchwork-Id: 12868063 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 D7912C433EF for ; Thu, 2 Jun 2022 18:54:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238355AbiFBSy1 (ORCPT ); Thu, 2 Jun 2022 14:54:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42022 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238312AbiFBSyY (ORCPT ); Thu, 2 Jun 2022 14:54:24 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id EBA731339C4 for ; Thu, 2 Jun 2022 11:54:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1654196056; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=1q/cIPahB7x9FEDPxFfUSaoR08YIuoc7nswOzy1p8SY=; b=Luj8ZxFf0vow/vAZnUf3eU2xLEUMQX4aAvMG1DLTWcb+lcBu537tDTYr+61Z0ViPiAIS+k i4at7hVouAbpzddEkJbJdX1e8rYZcA3qihIkvN5YmtoeoMXGf7/Mncxs4HUvT+A3VupVJp aKU+1kBTtISTTZaDHI49xGe20w7N2bs= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-386-HDguoosZNKaZD2O6D2Drww-1; Thu, 02 Jun 2022 14:54:14 -0400 X-MC-Unique: HDguoosZNKaZD2O6D2Drww-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7FB43101043B; Thu, 2 Jun 2022 18:54:14 +0000 (UTC) Received: from llong.com (unknown [10.22.32.147]) by smtp.corp.redhat.com (Postfix) with ESMTP id 325581121314; Thu, 2 Jun 2022 18:54:14 +0000 (UTC) From: Waiman Long To: Tejun Heo , Jens Axboe Cc: cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Ming Lei , Waiman Long Subject: [PATCH v5 4/4] blk-cgroup: Document the design of new lockless iostat_cpu list Date: Thu, 2 Jun 2022 14:54:01 -0400 Message-Id: <20220602185401.162937-1-longman@redhat.com> In-Reply-To: <20220602133543.128088-2-longman@redhat.com> References: <20220602133543.128088-2-longman@redhat.com> MIME-Version: 1.0 Content-type: text/plain X-Scanned-By: MIMEDefang 2.78 on 10.11.54.3 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org A set of percpu lockless lists per block cgroup (blkcg) is added to track the set of recently updated iostat_cpu structures. Add comment in the code to document the design of this new set of lockless lists. Signed-off-by: Waiman Long Acked-by: Tejun Heo --- block/blk-cgroup.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c index 8af97f3b2fc9..f8f27551c16a 100644 --- a/block/blk-cgroup.c +++ b/block/blk-cgroup.c @@ -60,6 +60,21 @@ static struct workqueue_struct *blkcg_punt_bio_wq; #define BLKG_DESTROY_BATCH_SIZE 64 /* + * Lockless lists for tracking IO stats update + * + * New IO stats are stored in the percpu iostat_cpu within blkcg_gq (blkg). + * There are multiple blkg's (one for each block device) attached to each + * blkcg. The rstat code keeps track of which cpu has IO stats updated, + * but it doesn't know which blkg has the updated stats. If there are many + * block devices in a system, the cost of iterating all the blkg's to flush + * out the IO stats can be high. To reduce such overhead, a set of percpu + * lockless lists (lhead) per blkcg are used to track the set of recently + * updated iostat_cpu's since the last flush. An iostat_cpu will be put + * onto the lockless list on the update side [blk_cgroup_bio_start()] if + * not there yet and then removed when being flushed [blkcg_rstat_flush()]. + * References to blkg are gotten and then put back in the process to + * protect against blkg removal. + * * lnode.next of the last entry in a lockless list is NULL. To enable us to * use lnode.next as a boolean flag to indicate its presence in a lockless * list, we have to make it non-NULL for all. This is done by using a