From patchwork Tue Sep 14 00:13:04 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: NeilBrown X-Patchwork-Id: 12491083 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 84947C43219 for ; Tue, 14 Sep 2021 00:28:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6AF0060FBF for ; Tue, 14 Sep 2021 00:28:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238372AbhINA3T (ORCPT ); Mon, 13 Sep 2021 20:29:19 -0400 Received: from smtp-out2.suse.de ([195.135.220.29]:55074 "EHLO smtp-out2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238305AbhINA3S (ORCPT ); Mon, 13 Sep 2021 20:29:18 -0400 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 94D4E200AA; Tue, 14 Sep 2021 00:28:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1631579280; h=from:from:reply-to: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=fz478wuK2aqSUdVMTm0N58JnzuLLXcDfeQcH+i0TRAc=; b=xe9UJcFJ9/etWuZVpkedbCfZdZGh4I+9k/xRiVFR+eOLjUc/38MzmSICzhxCaBDieHWCCU IGfVi+k1JTkaS1tbSmYAM2N1g3fFIk1xYp+Z9Ww/PkVOo0+PW5yrER7llpNQuvMk4su7zT Bx3yl4AFRjURJB8UHQZi0ijxP89K9dA= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1631579280; h=from:from:reply-to: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=fz478wuK2aqSUdVMTm0N58JnzuLLXcDfeQcH+i0TRAc=; b=Z5hLlNcuwJJinzCF56fW+4/hhbShZz4exvm5VcH5q6dN4x9juqo28ZwRLP51q89RkWh1Kh Pld6ww2EgjdayQDQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 386E713ADE; Tue, 14 Sep 2021 00:27:56 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id 5x0MOozsP2EiawAAMHmgww (envelope-from ); Tue, 14 Sep 2021 00:27:56 +0000 From: NeilBrown To: Andrew Morton , Theodore Ts'o , Andreas Dilger , "Darrick J. Wong" , Matthew Wilcox , Mel Gorman Date: Tue, 14 Sep 2021 10:13:04 +1000 Subject: [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective. Cc: linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Message-ID: <163157838437.13293.15392955714346973750.stgit@noble.brown> In-Reply-To: <163157808321.13293.486682642188075090.stgit@noble.brown> References: <163157808321.13293.486682642188075090.stgit@noble.brown> User-Agent: StGit/0.23 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Only 4 subsystems call set_bdi_congested() or clear_bdi_congested(): block/pktcdvd, fs/ceph fs/fuse fs/nfs It may make sense to use congestion_wait() or wait_iff_congested() within these subsystems, but they have no value outside of these. Add documentation comments to these functions to discourage further use. Signed-off-by: NeilBrown --- include/linux/backing-dev.h | 7 +++++++ mm/backing-dev.c | 9 +++++++++ 2 files changed, 16 insertions(+) diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h index ac7f231b8825..cc9513840351 100644 --- a/include/linux/backing-dev.h +++ b/include/linux/backing-dev.h @@ -153,6 +153,13 @@ static inline int wb_congested(struct bdi_writeback *wb, int cong_bits) return wb->congested & cong_bits; } +/* NOTE congestion_wait() and wait_iff_congested() are + * largely useless except as documentation. + * congestion_wait() will (almost) always wait for the given timeout. + * wait_iff_congested() will (almost) never wait, but will call + * cond_resched(). + * Were possible an alternative waiting strategy should be found. + */ long congestion_wait(int sync, long timeout); long wait_iff_congested(int sync, long timeout); diff --git a/mm/backing-dev.c b/mm/backing-dev.c index 4a9d4e27d0d9..53472ab38796 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -1023,6 +1023,11 @@ EXPORT_SYMBOL(set_bdi_congested); * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit * write congestion. If no backing_devs are congested then just wait for the * next write to be completed. + * + * NOTE: in the current implementation, hardly any backing_devs are ever + * marked as congested, and write-completion is rarely reported (see calls + * to clear_bdi_congested). So this should not be assumed to ever wake before + * the timeout. */ long congestion_wait(int sync, long timeout) { @@ -1054,6 +1059,10 @@ EXPORT_SYMBOL(congestion_wait); * The return value is 0 if the sleep is for the full timeout. Otherwise, * it is the number of jiffies that were still remaining when the function * returned. return_value == timeout implies the function did not sleep. + * + * NOTE: in the current implementation, hardly any backing_devs are ever + * marked as congested, and write-completion is rarely reported (see calls + * to clear_bdi_congested). So this should not be assumed to sleep at all. */ long wait_iff_congested(int sync, long timeout) {