From patchwork Fri May 27 15:28:17 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Coly Li X-Patchwork-Id: 12863424 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 AC462C4332F for ; Fri, 27 May 2022 15:32:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1353619AbiE0PcD (ORCPT ); Fri, 27 May 2022 11:32:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42458 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1353408AbiE0PcC (ORCPT ); Fri, 27 May 2022 11:32:02 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5E68F2A706; Fri, 27 May 2022 08:32:00 -0700 (PDT) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 5D00E21A96; Fri, 27 May 2022 15:31:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1653665519; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=mGAm6XwUC21DZwBnhAQtATzQHpXHm4XfYMxwjjPoB3E=; b=ARz6kjuV/zV4yTRWaHhi1bUsAi/PwC5UeN3+/r8iNqtuT9D1J1efQkHNBbNRi5qhbv71Dm T04VNgs8TlKGqFECSOLC1GHo+7Zz17byVXMxtmFs6TANhAl8Wkoxw/DlNPgWYngbZQjgIt 4OYLJx/T4DJ6hChNKQ9mzzzFkTtzpzA= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1653665519; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=mGAm6XwUC21DZwBnhAQtATzQHpXHm4XfYMxwjjPoB3E=; b=6Zt0dnfUW3iKrp7BVuWN5a5MM44RHIci5cJBADHAIH2XjMMlruc7mcKBvnENjMVJ73OFdA TPxTpSPnXghvW5Cg== Received: from localhost.localdomain (unknown [10.163.16.22]) by relay2.suse.de (Postfix) with ESMTP id 6927F2C141; Fri, 27 May 2022 15:31:44 +0000 (UTC) From: Coly Li To: axboe@kernel.dk Cc: linux-bcache@vger.kernel.org, linux-block@vger.kernel.org, Coly Li Subject: [PATCH 2/3] bcache: avoid unnecessary soft lockup in kworker update_writeback_rate() Date: Fri, 27 May 2022 23:28:17 +0800 Message-Id: <20220527152818.27545-3-colyli@suse.de> X-Mailer: git-send-email 2.35.3 In-Reply-To: <20220527152818.27545-1-colyli@suse.de> References: <20220527152818.27545-1-colyli@suse.de> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org The kworker routine update_writeback_rate() is schedued to update the writeback rate in every 5 seconds by default. Before calling __update_writeback_rate() to do real job, semaphore dc->writeback_lock should be held by the kworker routine. At the same time, bcache writeback thread routine bch_writeback_thread() also needs to hold dc->writeback_lock before flushing dirty data back into the backing device. If the dirty data set is large, it might be very long time for bch_writeback_thread() to scan all dirty buckets and releases dc->writeback_lock. In such case update_writeback_rate() can be starved for long enough time so that kernel reports a soft lockup warn- ing started like: watchdog: BUG: soft lockup - CPU#246 stuck for 23s! [kworker/246:31:179713] Such soft lockup condition is unnecessary, because after the writeback thread finishes its job and releases dc->writeback_lock, the kworker update_writeback_rate() may continue to work and everything is fine indeed. This patch avoids the unnecessary soft lockup by the following method, - Add new members to struct cached_dev - dc->retry_nr (0 by default) - dc->retry_max (5 by default) - In update_writeback_rate() call down_read_trylock(&dc->writeback_lock) firstly, if it fails then lock contention happens. Then dc->retry_nr adds 1, and mark contention as true - If dc->retry_nr <= dc->retry_max, give up updating the writeback rate and reschedules the kworker to retry after a longer time. - If dc->retry_nr > dc->retry_max, no retry anymore and call down_read(&dc->writeback_lock) to wait for the lock. By the above method, at worst case update_writeback_rate() may retry for 1+ minutes before blocking on dc->writeback_lock by calling down_read(). For a 4TB cache device with 1TB dirty data, 90%+ of the unnecessary soft lockup warning message can be avoided. When retrying to acquire dc->writeback_lock in update_writeback_rate(), of course the writeback rate cannot be updated. It is fair, because when the kworker is blocked on the lock contention of dc->writeback_lock, the writeback rate cannot be updated neither. Signed-off-by: Coly Li --- drivers/md/bcache/bcache.h | 7 ++++++ drivers/md/bcache/writeback.c | 45 +++++++++++++++++++++++++++++++---- 2 files changed, 47 insertions(+), 5 deletions(-) diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h index 9ed9c955add7..82b86b874294 100644 --- a/drivers/md/bcache/bcache.h +++ b/drivers/md/bcache/bcache.h @@ -395,6 +395,13 @@ struct cached_dev { atomic_t io_errors; unsigned int error_limit; unsigned int offline_seconds; + + /* + * Retry to update writeback_rate if contention happens for + * down_read(dc->writeback_lock) in update_writeback_rate() + */ + unsigned int retry_nr; + unsigned int retry_max; }; enum alloc_reserve { diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c index d138a2d73240..c51671abe74e 100644 --- a/drivers/md/bcache/writeback.c +++ b/drivers/md/bcache/writeback.c @@ -214,6 +214,7 @@ static void update_writeback_rate(struct work_struct *work) struct cached_dev, writeback_rate_update); struct cache_set *c = dc->disk.c; + bool contention = false; /* * should check BCACHE_DEV_RATE_DW_RUNNING before calling @@ -243,13 +244,41 @@ static void update_writeback_rate(struct work_struct *work) * in maximum writeback rate number(s). */ if (!set_at_max_writeback_rate(c, dc)) { - down_read(&dc->writeback_lock); - __update_writeback_rate(dc); - update_gc_after_writeback(c); - up_read(&dc->writeback_lock); + /* + * When contention happens on dc->writeback_lock with + * the writeback thread, this kwork may be blocked for + * very long time if there are too many dirty data to + * writeback, and kerne message will complain a (bogus) + * software lockup kernel message. To avoid potential + * starving, if down_read_trylock() fails, writeback + * rate updating will be skipped for dc->retry_max times + * at most while delay this worker a bit longer time. + * If dc->retry_max times are tried and the trylock + * still fails, then call down_read() to wait for + * dc->writeback_lock. + */ + if (!down_read_trylock((&dc->writeback_lock))) { + contention = true; + dc->retry_nr++; + if (dc->retry_nr > dc->retry_max) + down_read(&dc->writeback_lock); + } + + if (!contention || dc->retry_nr > dc->retry_max) { + __update_writeback_rate(dc); + update_gc_after_writeback(c); + up_read(&dc->writeback_lock); + dc->retry_nr = 0; + } } } + /* + * In case no lock contention on dc->writeback_lock happens since + * last retry, e.g. cache is clean or I/O idle for a while. + */ + if (!contention && dc->retry_nr) + dc->retry_nr = 0; /* * CACHE_SET_IO_DISABLE might be set via sysfs interface, @@ -257,8 +286,10 @@ static void update_writeback_rate(struct work_struct *work) */ if (test_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags) && !test_bit(CACHE_SET_IO_DISABLE, &c->flags)) { + unsigned int scale = 1 + dc->retry_nr; + schedule_delayed_work(&dc->writeback_rate_update, - dc->writeback_rate_update_seconds * HZ); + dc->writeback_rate_update_seconds * scale * HZ); } /* @@ -1006,6 +1037,10 @@ void bch_cached_dev_writeback_init(struct cached_dev *dc) dc->writeback_rate_fp_term_high = 1000; dc->writeback_rate_i_term_inverse = 10000; + /* For dc->writeback_lock contention in update_writeback_rate() */ + dc->retry_nr = 0; + dc->retry_max = 5; + WARN_ON(test_and_clear_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags)); INIT_DELAYED_WORK(&dc->writeback_rate_update, update_writeback_rate); }