From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-133.freemail.mail.aliyun.com (out30-133.freemail.mail.aliyun.com [115.124.30.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8389915665C; Mon, 18 May 2026 11:14:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.133 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779102907; cv=none; b=CGc24MtVjaErhJLUkl3FLOR7HoUFh936I9BdKGpMxiE9CFTLihwL7AFF88yEHsFgoInEGKL30gAY+1u194/62aHGvwz/hFYGotHG8HnGERjqAp6JoZk8D+gTvuMLXzHpmOstR8ZzDbuc32FN7YVxp1YDbKLzQHoBPd013k5BpwI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779102907; c=relaxed/simple; bh=KhNpJbVIUlx21LEZGn3F96IditzuTXLx3ECJUvpCKf4=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=bD/jnfwk9Y28AbdyzE6f/q4iVFjvom5SuSyrgnVFMot6pBmyTCbUT4KA0J+7p0dr3ybmcVuHKk4V2jiyum4qWbKIBltwi3ktHOmT6O1mpdtVmA4L+UFwq4HKIv6pzOosYYns5BAshhJMHu8o0HT7ozjFLm3uhEkzEEgFsTwpxZI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=VgXlocFJ; arc=none smtp.client-ip=115.124.30.133 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="VgXlocFJ" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1779102884; h=Message-ID:Date:MIME-Version:Subject:From:To:Content-Type; bh=R/4+8CHsIFQEAoNMVUEWELSlUHr6tUOtFVDHvmNzzWE=; b=VgXlocFJ38m059jjTzH2J8DhCa41FiEJ3FB3VwI4ZePE8/b52v3a70EXFHsbWgeuhKj53b5ScxGG27/V1NmtJWrl2ipimTtlr20klsK9fkL87x6VqIgxEYsNSJRLpBlUiuqLJcnyl58vr177ze1ANTLG1HHjVcb5HqJ2rYqJK60= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R151e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam011083073210;MF=libaokun@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0X37czye_1779102883; Received: from 30.221.131.155(mailfrom:libaokun@linux.alibaba.com fp:SMTPD_---0X37czye_1779102883 cluster:ay36) by smtp.aliyun-inc.com; Mon, 18 May 2026 19:14:43 +0800 Message-ID: Date: Mon, 18 May 2026 19:14:42 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/3] writeback: use a per-sb counter to drain inode wb switches at umount From: Baokun Li To: linux-fsdevel@vger.kernel.org Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, tj@kernel.org, linux-kernel@vger.kernel.org References: <20260517142147.3354909-1-libaokun@linux.alibaba.com> <20260517142147.3354909-4-libaokun@linux.alibaba.com> In-Reply-To: <20260517142147.3354909-4-libaokun@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Please ignore this patch; sashiko found some issues, and I will use wait_var_event() / wake_up_var() in the next version. https://sashiko.dev/#/patchset/20260517142147.3354909-1-libaokun%40linux.alibaba.com Sorry for the noise. Regards, Baokun 在 2026/5/17 22:21, Baokun Li 写道: > Tracking in-flight inode wb switches with a single global counter > (isw_nr_in_flight) plus a synchronize_rcu() based wait in > cgroup_writeback_umount() forces every umount to take a global hit > whenever any other superblock on the system has wb switches in flight, > even if the superblock being unmounted has none of its own. > > Replace the global synchronize_rcu()/flush_workqueue() pair with a > per-sb counter, s_isw_nr_in_flight: > > - inode_prepare_wbs_switch() increments before checking SB_ACTIVE > and grabbing the inode; failure paths decrement before returning. > - process_inode_switch_wbs() decrements after the matching iput(). > - cgroup_writeback_umount() waits for the per-sb counter to reach zero. > > The smp_mb() pair between inode_prepare_wbs_switch() and > cgroup_writeback_umount() keeps the SB_ACTIVE / counter ordering: > either the umounter sees a non-zero counter and waits, or the switcher > sees SB_ACTIVE cleared and aborts before grabbing the inode. > > The existing global isw_nr_in_flight is left in place, since it is > still used to throttle in-flight switches via WB_FRN_MAX_IN_FLIGHT. > > The rcu_read_lock() extension in inode_switch_wbs() and > cleanup_offline_cgwb() introduced by the race fix is no longer > needed and is reverted. The synchronize_rcu() in > cgroup_writeback_umount() is dropped as well. > > inode_prepare_wbs_switch() also gains a lockless SB_ACTIVE check at > the top to skip the atomic_inc/smp_mb cost for inodes whose sb is > already being torn down. > > The following numbers were measured with 4 background superblocks > each churning "create memcg -> write 1 MiB -> rmdir memcg" to keep > the global isw_nr_in_flight non-zero. A separate idle target sb is > mounted and umounted in a loop (N=100); only its umount latency is > measured. > > Idle target umount latency under cross-sb cgwb-switch pressure: > > p50 p95 p99 max > global synchronize_rcu() 64.4 ms 95.8 ms 101.4 ms 110.5 ms > per-sb counter (this) 5.3 ms 6.9 ms 7.4 ms 7.7 ms > no-pressure baseline 5.2 ms 5.9 ms 6.0 ms 6.1 ms > > 8 concurrent umounts of idle sbs under the same pressure: > > p50 p95 max > global synchronize_rcu() 57.9 ms 82.1 ms 90.0 ms > per-sb counter (this) 7.5 ms 7.8 ms 8.0 ms > > In-kernel cgroup_writeback_umount() time over 286 calls (bpftrace): > > global synchronize_rcu() 8717 ms total (~30 ms / call) > per-sb counter (this) 1.16 ms total (~4 us / call) > > When s_isw_nr_in_flight is zero the loop body is never entered. > > Signed-off-by: Baokun Li > --- > fs/fs-writeback.c | 74 ++++++++++++++-------------------- > include/linux/fs/super_types.h | 8 ++++ > 2 files changed, 38 insertions(+), 44 deletions(-) > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > index 9ae290547eb2..4282fcfe027b 100644 > --- a/fs/fs-writeback.c > +++ b/fs/fs-writeback.c > @@ -554,8 +554,12 @@ static void process_inode_switch_wbs(struct bdi_writeback *new_wb, > wb_put_many(old_wb, nr_switched); > } > > - for (inodep = isw->inodes; *inodep; inodep++) > + for (inodep = isw->inodes; *inodep; inodep++) { > + struct super_block *sb = (*inodep)->i_sb; > + > iput(*inodep); > + atomic_dec(&sb->s_isw_nr_in_flight); > + } > wb_put(new_wb); > kfree(isw); > atomic_dec(&isw_nr_in_flight); > @@ -598,16 +602,19 @@ void inode_switch_wbs_work_fn(struct work_struct *work) > static bool inode_prepare_wbs_switch(struct inode *inode, > struct bdi_writeback *new_wb) > { > + /* Avoid the atomic_inc/smp_mb dance once SB_ACTIVE is gone. */ > + if (!(inode->i_sb->s_flags & SB_ACTIVE)) > + return false; > + > /* > - * Paired with smp_mb() in cgroup_writeback_umount(). > - * isw_nr_in_flight must be increased before checking SB_ACTIVE and > - * grabbing an inode, otherwise isw_nr_in_flight can be observed as 0 > - * in cgroup_writeback_umount() and the isw_wq will be not flushed. > + * Pairs with smp_mb() in cgroup_writeback_umount(): the umounter either > + * sees a non-zero counter and waits, or we see SB_ACTIVE clear below. > */ > + atomic_inc(&inode->i_sb->s_isw_nr_in_flight); > smp_mb(); > > if (IS_DAX(inode)) > - return false; > + goto out_dec; > > /* while holding I_WB_SWITCH, no one else can update the association */ > spin_lock(&inode->i_lock); > @@ -615,13 +622,17 @@ static bool inode_prepare_wbs_switch(struct inode *inode, > inode_state_read(inode) & (I_WB_SWITCH | I_FREEING | I_WILL_FREE) || > inode_to_wb(inode) == new_wb) { > spin_unlock(&inode->i_lock); > - return false; > + goto out_dec; > } > inode_state_set(inode, I_WB_SWITCH); > __iget(inode); > spin_unlock(&inode->i_lock); > > return true; > + > +out_dec: > + atomic_dec(&inode->i_sb->s_isw_nr_in_flight); > + return false; > } > > static void wb_queue_isw(struct bdi_writeback *wb, > @@ -665,6 +676,7 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) > memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys); > if (memcg_css && !css_tryget(memcg_css)) > memcg_css = NULL; > + rcu_read_unlock(); > if (!memcg_css) > goto out_free; > > @@ -679,18 +691,10 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) > isw->inodes[0] = inode; > > trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1); > - /* > - * Paired with synchronize_rcu() in cgroup_writeback_umount(). > - * Holding rcu_read_lock across wb_queue_isw() ensures > - * synchronize_rcu() cannot return until the work is queued, so > - * the subsequent flush_workqueue() will wait for the switch. > - */ > wb_queue_isw(new_wb, isw); > - rcu_read_unlock(); > return; > > out_free: > - rcu_read_unlock(); > atomic_dec(&isw_nr_in_flight); > if (new_wb) > wb_put(new_wb); > @@ -748,14 +752,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb) > new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */ > > nr = 0; > - /* > - * Paired with synchronize_rcu() in cgroup_writeback_umount(). > - * Holding rcu_read_lock across the SB_ACTIVE check, the inode grab > - * and wb_queue_isw() ensures synchronize_rcu() cannot return until > - * the work is queued, so the subsequent flush_workqueue() will wait > - * for the switch. > - */ > - rcu_read_lock(); > spin_lock(&wb->list_lock); > /* > * In addition to the inodes that have completed writeback, also switch > @@ -773,7 +769,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb) > > /* no attached inodes? bail out */ > if (nr == 0) { > - rcu_read_unlock(); > atomic_dec(&isw_nr_in_flight); > wb_put(new_wb); > kfree(isw); > @@ -782,7 +777,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb) > > trace_inode_switch_wbs_queue(wb, new_wb, nr); > wb_queue_isw(new_wb, isw); > - rcu_read_unlock(); > > return restart; > } > @@ -1215,38 +1209,30 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id, > } > > /** > - * cgroup_writeback_umount - flush inode wb switches for umount > + * cgroup_writeback_umount - wait for in-flight inode wb switches on @sb > * @sb: target super_block > * > - * This function is called when a super_block is about to be destroyed and > - * flushes in-flight inode wb switches. An inode wb switch goes through > - * RCU and then workqueue, so the two need to be flushed in order to ensure > - * that all previously scheduled switches are finished. As wb switches are > - * rare occurrences and synchronize_rcu() can take a while, perform > - * flushing iff wb switches are in flight. > + * Wait until every inode wb switch that already passed the SB_ACTIVE > + * check on this superblock has been completed by the worker. Since > + * SB_ACTIVE is cleared before this is called, no new switches can start > + * for @sb, so s_isw_nr_in_flight will monotonically drop to zero. > */ > void cgroup_writeback_umount(struct super_block *sb) > { > - > if (!(sb->s_bdi->capabilities & BDI_CAP_WRITEBACK)) > return; > > /* > - * SB_ACTIVE should be reliably cleared before checking > - * isw_nr_in_flight, see generic_shutdown_super(). > + * Pairs with smp_mb() in inode_prepare_wbs_switch(): we either observe > + * a non-zero counter and wait, or the switcher sees SB_ACTIVE clear > + * (cleared by generic_shutdown_super()) and bails before grabbing the > + * inode. > */ > smp_mb(); > > - if (atomic_read(&isw_nr_in_flight)) { > - /* > - * Paired with rcu_read_lock() in inode_switch_wbs() and > - * cleanup_offline_cgwb(). synchronize_rcu() waits for any > - * in-flight switcher that already passed the SB_ACTIVE check > - * to finish queueing its work, so flush_workqueue() below > - * will then drain it. > - */ > - synchronize_rcu(); > + while (atomic_read(&sb->s_isw_nr_in_flight)) { > flush_workqueue(isw_wq); > + cond_resched(); > } > } > > diff --git a/include/linux/fs/super_types.h b/include/linux/fs/super_types.h > index 383050e7fdf5..1ab4e2265129 100644 > --- a/include/linux/fs/super_types.h > +++ b/include/linux/fs/super_types.h > @@ -274,6 +274,14 @@ struct super_block { > > /* number of fserrors that are being sent to fsnotify/filesystems */ > refcount_t s_pending_errors; > + > +#ifdef CONFIG_CGROUP_WRITEBACK > + /* > + * Number of in-flight inode wb switches for this sb. Drained by > + * cgroup_writeback_umount() before tear-down. > + */ > + atomic_t s_isw_nr_in_flight; > +#endif > } __randomize_layout; > > /*