From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-111.freemail.mail.aliyun.com (out30-111.freemail.mail.aliyun.com [115.124.30.111]) (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 B9A441DD877; Tue, 19 May 2026 06:33:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.111 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779172411; cv=none; b=aj2pPRi412KX/f2ZnQl5IXTSXCj0lqvsqWUqoZckmVouvg2VHFJKYmSUwXkYjOIVG2fQ1g3IlKkWforz9dlRY3TYPlWbjmywv4G2hf2an5TDLoXu1XeOaYqN2zqd/kBMWhJeFX6NPtaVyvWDHjj3RhnUtuQyVwU80StWj8CjU2o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779172411; c=relaxed/simple; bh=6CUgQj3y2qR30UKz1mrl2MpLvMVX9JQhCFCTnTdLzik=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=J9FL3hUuYU8Lizjkdu/kNlJAwDmdER756z82OU13Shhi/z47JGNjgbEsHJWLSn836fGE8kGe/P6kaj++vX6z5PEX9EqETVsqPptlbODTagvr+6mV9nJZXSp11zeps2tcPg4R2U8I4+WIXYGDwvQ0vyZXnyIXuJ9bfhj/9QfF17E= 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=Qwj/F+04; arc=none smtp.client-ip=115.124.30.111 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="Qwj/F+04" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1779172398; h=Message-ID:Date:MIME-Version:Subject:From:To:Content-Type; bh=tuGeYJn5JPpwLCyrkrmZKvODb16bs3XCOjGTVAPoWhI=; b=Qwj/F+04YWBqgxNKwUEq6vFl8///APVS2Jt0T4Oa3BjK71hoP/gRoajUPhySCH/UtW2MSvQ0m40gTNQeexMZVZEpUkkHauJ/Hp0sm1DtdSdcQHOGL4AIXKzxWp7SrrB7gWp4DqIqNeprvCK8npZCsALDg9/NlsL870sWGbVrPgo= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R371e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033037033178;MF=libaokun@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0X3ElH3Z_1779172397; Received: from 30.221.134.159(mailfrom:libaokun@linux.alibaba.com fp:SMTPD_---0X3ElH3Z_1779172397 cluster:ay36) by smtp.aliyun-inc.com; Tue, 19 May 2026 14:33:17 +0800 Message-ID: Date: Tue, 19 May 2026 14:33:17 +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 v3 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: <20260518135349.1187628-1-libaokun@linux.alibaba.com> <20260518135349.1187628-4-libaokun@linux.alibaba.com> In-Reply-To: <20260518135349.1187628-4-libaokun@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sashiko[1] flagged a stale comment in patch 3 — the rcu_read_lock() comment still describes the old synchronize_rcu()/flush_workqueue() scheme which no longer exists after this patch. I'll fold the following fixup into the next version: --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -688,16 +688,8 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)           atomic_inc(&isw_nr_in_flight);   -       /* -        * Paired with synchronize_rcu() in cgroup_writeback_umount(): -        * holding rcu_read_lock across inode_prepare_wbs_switch() -        * (covering the SB_ACTIVE check and 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();         /* find and pin the new wb */ +       rcu_read_lock();         memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);         if (memcg_css && !css_tryget(memcg_css))                 memcg_css = NULL; In no hurry to resend — further review is welcome. [1] https://sashiko.dev/#/patchset/20260518135349.1187628-1-libaokun%40linux.alibaba.com Thanks, Baokun On 2026/5/18 21:53, Baokun Li wrote: > 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, plus three small helpers: > > - cgroup_writeback_pin(sb) - increment counter > - cgroup_writeback_unpin(sb) - decrement and wake drainer if last > - cgroup_writeback_drain(sb) - wait for counter to reach zero > > The wiring is: > > - inode_prepare_wbs_switch() pins before checking SB_ACTIVE and > grabbing the inode; failure paths unpin before returning. A > lockless SB_ACTIVE check at the top of the function lets us skip > the atomic_inc/smp_mb dance once SB_ACTIVE has been cleared (it > is monotonic and never set back). > - process_inode_switch_wbs() unpins after the matching iput(). > - cgroup_writeback_umount() drains the per-sb counter via > wait_var_event(). > > 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 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() that the race fix added is no longer needed > and is reverted; the synchronize_rcu() that the race fix added to > cgroup_writeback_umount() is dropped as well. > > The following numbers were measured on a 16 vCPU QEMU guest with 4 > background superblocks each churning "create memcg -> write 1 MiB -> > rmdir memcg" to keep the global isw_nr_in_flight non-zero. Latencies > are wall-clock around umount(8); only the target sb's umount is > measured. > > Target sb runs its own cgwb churn: > > p50 p95 p99 max > global synchronize_rcu() 67.6 ms 88.3 ms 88.3 ms 96.8 ms > per-sb counter (this) 7.9 ms 10.0 ms 10.0 ms 10.1 ms > > Idle target umount latency under cross-sb cgwb-switch pressure: > > p50 p95 p99 max > global synchronize_rcu() 62.7 ms 95.4 ms 108.1 ms 108.6 ms > per-sb counter (this) 5.3 ms 6.9 ms 7.4 ms 7.4 ms > no-pressure baseline 4.9 ms 5.9 ms 6.3 ms 6.7 ms > > 8 concurrent umounts of idle sbs under the same pressure: > > p50 p95 max > global synchronize_rcu() 61.3 ms 99.5 ms 113.7 ms > per-sb counter (this) 8.1 ms 9.1 ms 9.5 ms > > In-kernel cgroup_writeback_umount() time across the same run > (bpftrace, ~340 calls covering all scenarios): > > global synchronize_rcu() 12371 ms total (~36 ms / call) > per-sb counter (this) 1.37 ms total ( ~4 us / call) > > Suggested-by: Christian Brauner > Link: https://lore.kernel.org/r/177910456953.488929.2169908940676707307.b4-review@b4 > Signed-off-by: Baokun Li > --- > fs/fs-writeback.c | 87 ++++++++++++++++++---------------- > include/linux/fs/super_types.h | 8 ++++ > 2 files changed, 54 insertions(+), 41 deletions(-) > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > index 325a30cc35bf..32fec4b9094e 100644 > --- a/fs/fs-writeback.c > +++ b/fs/fs-writeback.c > @@ -497,6 +497,23 @@ static bool inode_do_switch_wbs(struct inode *inode, > return switched; > } > > +static inline void cgroup_writeback_pin(struct super_block *sb) > +{ > + atomic_inc(&sb->s_isw_nr_in_flight); > +} > + > +static inline void cgroup_writeback_unpin(struct super_block *sb) > +{ > + if (atomic_dec_and_test(&sb->s_isw_nr_in_flight)) > + wake_up_var(&sb->s_isw_nr_in_flight); > +} > + > +static inline void cgroup_writeback_drain(struct super_block *sb) > +{ > + wait_var_event(&sb->s_isw_nr_in_flight, > + !atomic_read(&sb->s_isw_nr_in_flight)); > +} > + > static void process_inode_switch_wbs(struct bdi_writeback *new_wb, > struct inode_switch_wbs_context *isw) > { > @@ -554,8 +571,12 @@ 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); > + cgroup_writeback_unpin(sb); > + } > wb_put(new_wb); > kfree(isw); > atomic_dec(&isw_nr_in_flight); > @@ -598,16 +619,19 @@ 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. > */ > + cgroup_writeback_pin(inode->i_sb); > 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 +639,17 @@ 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: > + cgroup_writeback_unpin(inode->i_sb); > + return false; > } > > static void wb_queue_isw(struct bdi_writeback *wb, > @@ -673,6 +701,7 @@ 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; > > @@ -688,11 +717,9 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) > > trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1); > 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); > @@ -750,14 +777,6 @@ 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 > @@ -775,7 +794,6 @@ 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); > @@ -784,7 +802,6 @@ 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; > } > @@ -1217,39 +1234,27 @@ 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(); > - flush_workqueue(isw_wq); > - } > + cgroup_writeback_drain(sb); > } > > static int __init cgroup_writeback_init(void) > 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; > > /*