Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 21 Apr 2023 17:40:16 +0000
In-Reply-To: <20230421174020.2994750-1-yosryahmed@google.com>
Mime-Version: 1.0
References: <20230421174020.2994750-1-yosryahmed@google.com>
Message-ID: <20230421174020.2994750-2-yosryahmed@google.com>
Subject: [PATCH v5 1/5] writeback: move wb_over_bg_thresh() call outside lock section
From:   Yosry Ahmed <yosryahmed@google.com>
To:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>
Cc:     Jan Kara <jack@suse.cz>, Jens Axboe <axboe@kernel.dk>,
        "=?UTF-8?q?Michal=20Koutn=C3=BD?=" <mkoutny@suse.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        Yosry Ahmed <yosryahmed@google.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

wb_over_bg_thresh() calls mem_cgroup_wb_stats() which invokes an rstat
flush, which can be expensive on large systems. Currently,
wb_writeback() calls wb_over_bg_thresh() within a lock section, so we
have to do the rstat flush atomically. On systems with a lot of
cpus and/or cgroups, this can cause us to disable irqs for a long time,
potentially causing problems.

Move the call to wb_over_bg_thresh() outside the lock section in
preparation to make the rstat flush in mem_cgroup_wb_stats() non-atomic.
The list_empty(&wb->work_list) check should be okay outside the lock
section of wb->list_lock as it is protected by a separate lock
(wb->work_lock), and wb_over_bg_thresh() doesn't seem like it is
modifying any of wb->b_* lists the wb->list_lock is protecting.
Also, the loop seems to be already releasing and reacquring the
lock, so this refactoring looks safe.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Michal Koutn=C3=BD <mkoutny@suse.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---
 fs/fs-writeback.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 1db3e3c24b43..11aa1652fb84 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -2024,7 +2024,6 @@ static long wb_writeback(struct bdi_writeback *wb,
 	struct blk_plug plug;
=20
 	blk_start_plug(&plug);
-	spin_lock(&wb->list_lock);
 	for (;;) {
 		/*
 		 * Stop writeback when nr_pages has been consumed
@@ -2049,6 +2048,9 @@ static long wb_writeback(struct bdi_writeback *wb,
 		if (work->for_background && !wb_over_bg_thresh(wb))
 			break;
=20
+
+		spin_lock(&wb->list_lock);
+
 		/*
 		 * Kupdate and background works are special and we want to
 		 * include all inodes that need writing. Livelock avoidance is
@@ -2078,13 +2080,19 @@ static long wb_writeback(struct bdi_writeback *wb,
 		 * mean the overall work is done. So we keep looping as long
 		 * as made some progress on cleaning pages or inodes.
 		 */
-		if (progress)
+		if (progress) {
+			spin_unlock(&wb->list_lock);
 			continue;
+		}
+
 		/*
 		 * No more inodes for IO, bail
 		 */
-		if (list_empty(&wb->b_more_io))
+		if (list_empty(&wb->b_more_io)) {
+			spin_unlock(&wb->list_lock);
 			break;
+		}
+
 		/*
 		 * Nothing written. Wait for some inode to
 		 * become available for writeback. Otherwise
@@ -2096,9 +2104,7 @@ static long wb_writeback(struct bdi_writeback *wb,
 		spin_unlock(&wb->list_lock);
 		/* This function drops i_lock... */
 		inode_sleep_on_writeback(inode);
-		spin_lock(&wb->list_lock);
 	}
-	spin_unlock(&wb->list_lock);
 	blk_finish_plug(&plug);
=20
 	return nr_pages - work->nr_pages;
--=20
2.40.0.634.g4ca3ef3211-goog