From patchwork Sat May 28 12:45:50 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Coly Li X-Patchwork-Id: 12864043 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 1159DC433F5 for ; Sat, 28 May 2022 12:46:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235404AbiE1MqD (ORCPT ); Sat, 28 May 2022 08:46:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34486 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229734AbiE1MqC (ORCPT ); Sat, 28 May 2022 08:46:02 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8CEF0F8; Sat, 28 May 2022 05:46:01 -0700 (PDT) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 46ADA1F895; Sat, 28 May 2022 12:46:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1653741960; 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=IqQyIL5+3r+0aJMqJCo/t/VtD8yTF0XmgPhW311VF4o=; b=mltqktQZh3Zc3I7Wen7ecMtaOTjj8m+Whdzcsm2IrBxV9v9fTdKlRGBVdbaxOo+0+egIUu 4qj5j8Zt+R3CLu2NJVTCC4o+m4BYITPbc37/KMt9JCUxqndvYeEFh++YHf1A06y7lnaGZP XRjS/UyGqXhzxvrnm46M3yKq+15bwg8= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1653741960; 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=IqQyIL5+3r+0aJMqJCo/t/VtD8yTF0XmgPhW311VF4o=; b=AIa54PEm978USYp1CedMhEcOM7DDkYtuu7YXu9+IH0dDYxWb5yzfJ/ZibMhyO7xKV5MGSL izxmWhS48OGQahBg== Received: from localhost.localdomain (colyli.tcp.ovpn1.nue.suse.de [10.163.16.22]) by relay2.suse.de (Postfix) with ESMTP id 480AA2C141; Sat, 28 May 2022 12:45:57 +0000 (UTC) From: Coly Li To: axboe@kernel.dk Cc: linux-bcache@vger.kernel.org, linux-block@vger.kernel.org, Coly Li Subject: [PATCH 1/1] bcache: avoid unnecessary soft lockup in kworker update_writeback_rate() Date: Sat, 28 May 2022 20:45:50 +0800 Message-Id: <20220528124550.32834-2-colyli@suse.de> X-Mailer: git-send-email 2.35.3 In-Reply-To: <20220528124550.32834-1-colyli@suse.de> References: <20220528124550.32834-1-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 member to struct cached_dev - dc->rate_update_retry (0 by default) - In update_writeback_rate() call down_read_trylock(&dc->writeback_lock) firstly, if it fails then lock contention happens. - If dc->rate_update_retry <= BCH_WBRATE_UPDATE_MAX_SKIPS (15), doesn't acquire the lock and reschedules the kworker for next try. - If dc->rate_update_retry > BCH_WBRATE_UPDATE_MAX_SKIPS, 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. This change follows Jens Axboe's suggestion to a more clear and simple version. Signed-off-by: Coly Li --- drivers/md/bcache/bcache.h | 7 +++++++ drivers/md/bcache/writeback.c | 31 +++++++++++++++++++++---------- 2 files changed, 28 insertions(+), 10 deletions(-) diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h index 9ed9c955add7..2acda9cea0f9 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() + */ +#define BCH_WBRATE_UPDATE_MAX_SKIPS 15 + unsigned int rate_update_retry; }; enum alloc_reserve { diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c index d138a2d73240..3f0ff3aab6f2 100644 --- a/drivers/md/bcache/writeback.c +++ b/drivers/md/bcache/writeback.c @@ -235,19 +235,27 @@ static void update_writeback_rate(struct work_struct *work) return; } - if (atomic_read(&dc->has_dirty) && dc->writeback_percent) { - /* - * If the whole cache set is idle, set_at_max_writeback_rate() - * will set writeback rate to a max number. Then it is - * unncessary to update writeback rate for an idle cache set - * in maximum writeback rate number(s). - */ - if (!set_at_max_writeback_rate(c, dc)) { - down_read(&dc->writeback_lock); + /* + * If the whole cache set is idle, set_at_max_writeback_rate() + * will set writeback rate to a max number. Then it is + * unncessary to update writeback rate for an idle cache set + * in maximum writeback rate number(s). + */ + if (atomic_read(&dc->has_dirty) && dc->writeback_percent && + !set_at_max_writeback_rate(c, dc)) { + do { + if (!down_read_trylock((&dc->writeback_lock))) { + dc->rate_update_retry++; + if (dc->rate_update_retry <= + BCH_WBRATE_UPDATE_MAX_SKIPS) + break; + down_read(&dc->writeback_lock); + dc->rate_update_retry = 0; + } __update_writeback_rate(dc); update_gc_after_writeback(c); up_read(&dc->writeback_lock); - } + } while (0); } @@ -1006,6 +1014,9 @@ 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->rate_update_retry = 0; + WARN_ON(test_and_clear_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags)); INIT_DELAYED_WORK(&dc->writeback_rate_update, update_writeback_rate); }