Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755845AbZIDKyK (ORCPT ); Fri, 4 Sep 2009 06:54:10 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753559AbZIDKyJ (ORCPT ); Fri, 4 Sep 2009 06:54:09 -0400 Received: from cantor.suse.de ([195.135.220.2]:37599 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753480AbZIDKyG (ORCPT ); Fri, 4 Sep 2009 06:54:06 -0400 Date: Fri, 4 Sep 2009 12:54:03 +0200 From: Jan Kara To: Jens Axboe Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, chris.mason@oracle.com, david@fromorbit.com, hch@infradead.org, tytso@mit.edu, akpm@linux-foundation.org, jack@suse.cz Subject: Re: [PATCH 3/8] writeback: switch to per-bdi threads for flushing data Message-ID: <20090904105403.GD19857@duck.suse.cz> References: <1252050406-22467-1-git-send-email-jens.axboe@oracle.com> <1252050406-22467-4-git-send-email-jens.axboe@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1252050406-22467-4-git-send-email-jens.axboe@oracle.com> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 18576 Lines: 565 On Fri 04-09-09 09:46:41, Jens Axboe wrote: > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > index 45ad4bb..93aa9a7 100644 > --- a/fs/fs-writeback.c > +++ b/fs/fs-writeback.c ... > +static void wb_start_writeback(struct bdi_writeback *wb, struct bdi_work *work) > +{ > + /* > + * If we failed allocating the bdi work item, wake up the wb thread > + * always. As a safety precaution, it'll flush out everything > + */ > + if (!wb_has_dirty_io(wb) && work) > + wb_clear_pending(wb, work); > + else if (wb->task) > + wake_up_process(wb->task); > +} > > - inode->i_state |= flags; > +static void bdi_sched_work(struct backing_dev_info *bdi, struct bdi_work *work) > +{ > + wb_start_writeback(&bdi->wb, work); > +} wb_start_writeback gets called only from bdi_sched_work() that gets called only from bdi_queue_work(). I think it might be easier to read if we put everything in bdi_queue_work(). Also it's not quite clear to me, why wb_start_writeback() wakes up process even if wb_has_dirty_io() == 0. > +/* > + * For WB_SYNC_NONE writeback, the caller does not have the sb pinned > + * before calling writeback. So make sure that we do pin it, so it doesn't > + * go away while we are writing inodes from it. Maybe add here a comment that the function returns 0 if the sb pinned and 1 if it isn't (which seems slightly counterintuitive to me). > + */ > +static int pin_sb_for_writeback(struct writeback_control *wbc, > + struct inode *inode) > { > + struct super_block *sb = inode->i_sb; > + > + /* > + * Caller must already hold the ref for this > + */ > + if (wbc->sync_mode == WB_SYNC_ALL) { > + WARN_ON(!rwsem_is_locked(&sb->s_umount)); > + return 0; > + } > + > + spin_lock(&sb_lock); > + sb->s_count++; > + if (down_read_trylock(&sb->s_umount)) { > + if (sb->s_root) { > + spin_unlock(&sb_lock); > + return 0; > + } > + /* > + * umounted, drop rwsem again and fall through to failure > + */ > + up_read(&sb->s_umount); > + } > + > + __put_super_and_need_restart(sb); Here, you should be safe to do just sb->s_count-- since you didn't drop sb_lock in the mean time. Other > + spin_unlock(&sb_lock); > + return 1; > +} > + > +static void unpin_sb_for_writeback(struct writeback_control *wbc, > + struct inode *inode) > +{ > + struct super_block *sb = inode->i_sb; > + > + if (wbc->sync_mode == WB_SYNC_ALL) > + return; > + > + up_read(&sb->s_umount); > + spin_lock(&sb_lock); > + __put_super_and_need_restart(sb); > + spin_unlock(&sb_lock); Above three lines should be just: put_super(sb); > +} > + > +static void writeback_inodes_wb(struct bdi_writeback *wb, > + struct writeback_control *wbc) > +{ > + struct super_block *sb = wbc->sb; > const int is_blkdev_sb = sb_is_blkdev_sb(sb); > const unsigned long start = jiffies; /* livelock avoidance */ > > spin_lock(&inode_lock); > > - if (!wbc->for_kupdate || list_empty(&bdi->b_io)) > - queue_io(bdi, wbc->older_than_this); > + if (!wbc->for_kupdate || list_empty(&wb->b_io)) > + queue_io(wb, wbc->older_than_this); > > - while (!list_empty(&bdi->b_io)) { > - struct inode *inode = list_entry(bdi->b_io.prev, > + while (!list_empty(&wb->b_io)) { > + struct inode *inode = list_entry(wb->b_io.prev, > struct inode, i_list); > long pages_skipped; > > @@ -491,7 +559,7 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi, > continue; > } > > - if (!bdi_cap_writeback_dirty(bdi)) { > + if (!bdi_cap_writeback_dirty(wb->bdi)) { > redirty_tail(inode); > if (is_blkdev_sb) { > /* > @@ -513,7 +581,7 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi, > continue; > } > > - if (wbc->nonblocking && bdi_write_congested(bdi)) { > + if (wbc->nonblocking && bdi_write_congested(wb->bdi)) { > wbc->encountered_congestion = 1; > if (!is_blkdev_sb) > break; /* Skip a congested fs */ > @@ -521,13 +589,6 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi, > continue; /* Skip a congested blockdev */ > } > > - if (wbc->bdi && bdi != wbc->bdi) { > - if (!is_blkdev_sb) > - break; /* fs has the wrong queue */ > - requeue_io(inode); > - continue; /* blockdev has wrong queue */ > - } > - > /* > * Was this inode dirtied after sync_sb_inodes was called? > * This keeps sync from extra jobs and livelock. > @@ -535,16 +596,16 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi, > if (inode_dirtied_after(inode, start)) > break; > > - /* Is another pdflush already flushing this queue? */ > - if (current_is_pdflush() && !writeback_acquire(bdi)) > - break; > + if (pin_sb_for_writeback(wbc, inode)) { > + requeue_io(inode); > + continue; > + } > > BUG_ON(inode->i_state & (I_FREEING | I_CLEAR)); > __iget(inode); > pages_skipped = wbc->pages_skipped; > writeback_single_inode(inode, wbc); > - if (current_is_pdflush()) > - writeback_release(bdi); > + unpin_sb_for_writeback(wbc, inode); > if (wbc->pages_skipped != pages_skipped) { > /* > * writeback is not making progress due to locked This looks safe now. Good. > /* > - * Write out a superblock's list of dirty inodes. A wait will be performed > - * upon no inodes, all inodes or the final one, depending upon sync_mode. > - * > - * If older_than_this is non-NULL, then only write out inodes which > - * had their first dirtying at a time earlier than *older_than_this. > - * > - * If we're a pdlfush thread, then implement pdflush collision avoidance > - * against the entire list. > + * The maximum number of pages to writeout in a single bdi flush/kupdate > + * operation. We do this so we don't hold I_SYNC against an inode for > + * enormous amounts of time, which would block a userspace task which has > + * been forced to throttle against that inode. Also, the code reevaluates > + * the dirty each time it has written this many pages. > + */ > +#define MAX_WRITEBACK_PAGES 1024 > + > +static inline bool over_bground_thresh(void) > +{ > + unsigned long background_thresh, dirty_thresh; > + > + get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL); > + > + return (global_page_state(NR_FILE_DIRTY) + > + global_page_state(NR_UNSTABLE_NFS) >= background_thresh); > +} > + > +/* > + * Explicit flushing or periodic writeback of "old" data. > * > - * If `bdi' is non-zero then we're being asked to writeback a specific queue. > - * This function assumes that the blockdev superblock's inodes are backed by > - * a variety of queues, so all inodes are searched. For other superblocks, > - * assume that all inodes are backed by the same queue. > + * Define "old": the first time one of an inode's pages is dirtied, we mark the > + * dirtying-time in the inode's address_space. So this periodic writeback code > + * just walks the superblock inode list, writing back any inodes which are > + * older than a specific point in time. > * > - * FIXME: this linear search could get expensive with many fileystems. But > - * how to fix? We need to go from an address_space to all inodes which share > - * a queue with that address_space. (Easy: have a global "dirty superblocks" > - * list). > + * Try to run once per dirty_writeback_interval. But if a writeback event > + * takes longer than a dirty_writeback_interval interval, then leave a > + * one-second gap. > * > - * The inodes to be written are parked on bdi->b_io. They are moved back onto > - * bdi->b_dirty as they are selected for writing. This way, none can be missed > - * on the writer throttling path, and we get decent balancing between many > - * throttled threads: we don't want them all piling up on inode_sync_wait. > + * older_than_this takes precedence over nr_to_write. So we'll only write back > + * all dirty pages if they are all attached to "old" mappings. > */ > -static void generic_sync_sb_inodes(struct super_block *sb, > - struct writeback_control *wbc) > +static long wb_writeback(struct bdi_writeback *wb, long nr_pages, > + struct super_block *sb, > + enum writeback_sync_modes sync_mode, int for_kupdate) > { > - struct backing_dev_info *bdi; > + struct writeback_control wbc = { > + .bdi = wb->bdi, > + .sb = sb, > + .sync_mode = sync_mode, > + .older_than_this = NULL, > + .for_kupdate = for_kupdate, > + .range_cyclic = 1, > + }; > + unsigned long oldest_jif; > + long wrote = 0; > > - if (!wbc->bdi) { > - mutex_lock(&bdi_lock); > - list_for_each_entry(bdi, &bdi_list, bdi_list) > - generic_sync_bdi_inodes(bdi, wbc, sb); > - mutex_unlock(&bdi_lock); > - } else > - generic_sync_bdi_inodes(wbc->bdi, wbc, sb); > + if (wbc.for_kupdate) { > + wbc.older_than_this = &oldest_jif; > + oldest_jif = jiffies - > + msecs_to_jiffies(dirty_expire_interval * 10); > + } > > - if (wbc->sync_mode == WB_SYNC_ALL) { > - struct inode *inode, *old_inode = NULL; > + for (;;) { > + if (sync_mode == WB_SYNC_NONE && nr_pages <= 0 && > + !over_bground_thresh()) > + break; I don't understand this - why should this function care about over_bground_thresh? As I understand it, it should just do what it was told. For example when someone asks to write-out 20 pages from some superblock, we may effectively end up writing everyting from the superblock if the system happens to have another superblock with more than background_thresh of dirty pages... I guess you try to join pdflush-like and kupdate-like writeback here. Then you might want to have conditions like if (!for_kupdate && nr_pages <= 0 && sync_mode == WB_SYNC_NONE) break; if (for_kupdate && nr_pages <= 0 && !over_bground_thresh()) break; > - spin_lock(&inode_lock); > + wbc.more_io = 0; > + wbc.encountered_congestion = 0; > + wbc.nr_to_write = MAX_WRITEBACK_PAGES; > + wbc.pages_skipped = 0; > + writeback_inodes_wb(wb, &wbc); > + nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write; > + wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write; > > /* > - * Data integrity sync. Must wait for all pages under writeback, > - * because there may have been pages dirtied before our sync > - * call, but which had writeout started before we write it out. > - * In which case, the inode may not be on the dirty list, but > - * we still have to wait for that writeout. > + * If we ran out of stuff to write, bail unless more_io got set > */ > - list_for_each_entry(inode, &sb->s_inodes, i_sb_list) { > - struct address_space *mapping; > - > - if (inode->i_state & > - (I_FREEING|I_CLEAR|I_WILL_FREE|I_NEW)) > - continue; > - mapping = inode->i_mapping; > - if (mapping->nrpages == 0) > + if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) { > + if (wbc.more_io && !wbc.for_kupdate) > continue; > - __iget(inode); > - spin_unlock(&inode_lock); > + break; > + } > + } > + > + return wrote; > +} > + > +/* > + * Retrieve work items and do the writeback they describe > + */ > +long wb_do_writeback(struct bdi_writeback *wb, int force_wait) Why is here force_wait parameter? I don't see it being set anywhere... > +{ > + struct backing_dev_info *bdi = wb->bdi; > + struct bdi_work *work; > + long nr_pages, wrote = 0; > + > + while ((work = get_next_work_item(bdi, wb)) != NULL) { > + enum writeback_sync_modes sync_mode; > + > + nr_pages = work->nr_pages; > + > + /* > + * Override sync mode, in case we must wait for completion > + */ > + if (force_wait) > + work->sync_mode = sync_mode = WB_SYNC_ALL; > + else > + sync_mode = work->sync_mode; > + > + /* > + * If this isn't a data integrity operation, just notify > + * that we have seen this work and we are now starting it. > + */ > + if (sync_mode == WB_SYNC_NONE) > + wb_clear_pending(wb, work); > + > + wrote += wb_writeback(wb, nr_pages, work->sb, sync_mode, 0); > + > + /* > + * This is a data integrity writeback, so only do the > + * notification when we have completed the work. > + */ > + if (sync_mode == WB_SYNC_ALL) > + wb_clear_pending(wb, work); > + } > + > + /* > + * Check for periodic writeback, kupdated() style > + */ > + if (!wrote) { Hmm, but who guarantees that old inodes get flushed from dirty list when someone just periodically submits some work? And similarly who guarantees we drop below background threshold? I think the logic here needs some rethinking... > + nr_pages = global_page_state(NR_FILE_DIRTY) + > + global_page_state(NR_UNSTABLE_NFS) + > + (inodes_stat.nr_inodes - inodes_stat.nr_unused); > + > + wrote = wb_writeback(wb, nr_pages, NULL, WB_SYNC_NONE, 1); > + } > + > + return wrote; > +} > + > +/* > + * Handle writeback of dirty data for the device backed by this bdi. Also > + * wakes up periodically and does kupdated style flushing. > + */ > +int bdi_writeback_task(struct bdi_writeback *wb) > +{ > + unsigned long last_active = jiffies; > + unsigned long wait_jiffies = -1UL; > + long pages_written; > + > + while (!kthread_should_stop()) { > + pages_written = wb_do_writeback(wb, 0); > + > + if (pages_written) > + last_active = jiffies; > + else if (wait_jiffies != -1UL) { > + unsigned long max_idle; > + > /* > - * We hold a reference to 'inode' so it couldn't have > - * been removed from s_inodes list while we dropped the > - * inode_lock. We cannot iput the inode now as we can > - * be holding the last reference and we cannot iput it > - * under inode_lock. So we keep the reference and iput > - * it later. > + * Longest period of inactivity that we tolerate. If we > + * see dirty data again later, the task will get > + * recreated automatically. > */ > - iput(old_inode); > - old_inode = inode; > + max_idle = max(5UL * 60 * HZ, wait_jiffies); > + if (time_after(jiffies, max_idle + last_active)) > + break; > + } > + > + wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10); > + set_current_state(TASK_INTERRUPTIBLE); > + schedule_timeout(wait_jiffies); > + try_to_freeze(); > + } > > - filemap_fdatawait(mapping); > + return 0; > +} > > - cond_resched(); > +/* > + * Schedule writeback for all backing devices. Expensive! If this is a data > + * integrity operation, writeback will be complete when this returns. If > + * we are simply called for WB_SYNC_NONE, then writeback will merely be > + * scheduled to run. > + */ > +static void bdi_writeback_all(struct writeback_control *wbc) > +{ > + const bool must_wait = wbc->sync_mode == WB_SYNC_ALL; > + struct backing_dev_info *bdi; > + struct bdi_work *work; > + LIST_HEAD(list); > + > +restart: > + spin_lock(&bdi_lock); > + > + list_for_each_entry(bdi, &bdi_list, bdi_list) { > + struct bdi_work *work; > + > + if (!bdi_has_dirty_io(bdi)) > + continue; > > - spin_lock(&inode_lock); > + /* > + * If work allocation fails, do the writes inline. We drop > + * the lock and restart the list writeout. This should be OK, > + * since this happens rarely and because the writeout should > + * eventually make more free memory available. > + */ > + work = bdi_alloc_work(wbc); > + if (!work) { > + struct writeback_control __wbc = *wbc; > + > + /* > + * Not a data integrity writeout, just continue > + */ > + if (!must_wait) > + continue; > + > + spin_unlock(&bdi_lock); > + __wbc = *wbc; > + __wbc.bdi = bdi; > + writeback_inodes_wbc(&__wbc); > + goto restart; > } > - spin_unlock(&inode_lock); > - iput(old_inode); > + if (must_wait) > + list_add_tail(&work->wait_list, &list); > + > + bdi_queue_work(bdi, work); > + } > + > + spin_unlock(&bdi_lock); > + > + /* > + * If this is for WB_SYNC_ALL, wait for pending work to complete > + * before returning. > + */ > + while (!list_empty(&list)) { > + work = list_entry(list.next, struct bdi_work, wait_list); > + list_del(&work->wait_list); > + bdi_wait_on_work_clear(work); > + call_rcu(&work->rcu_head, bdi_work_free); > } > } ... > @@ -715,6 +1112,7 @@ restart: > long writeback_inodes_sb(struct super_block *sb) > { > struct writeback_control wbc = { > + .sb = sb, > .sync_mode = WB_SYNC_NONE, > .range_start = 0, > .range_end = LLONG_MAX, > @@ -727,7 +1125,7 @@ long writeback_inodes_sb(struct super_block *sb) > (inodes_stat.nr_inodes - inodes_stat.nr_unused); > > wbc.nr_to_write = nr_to_write; > - generic_sync_sb_inodes(sb, &wbc); > + bdi_writeback_all(&wbc); > return nr_to_write - wbc.nr_to_write; > } > EXPORT_SYMBOL(writeback_inodes_sb); > @@ -742,6 +1140,7 @@ EXPORT_SYMBOL(writeback_inodes_sb); > long sync_inodes_sb(struct super_block *sb) > { > struct writeback_control wbc = { > + .sb = sb, > .sync_mode = WB_SYNC_ALL, > .range_start = 0, > .range_end = LLONG_MAX, > @@ -749,7 +1148,8 @@ long sync_inodes_sb(struct super_block *sb) > long nr_to_write = LONG_MAX; /* doesn't actually matter */ > > wbc.nr_to_write = nr_to_write; > - generic_sync_sb_inodes(sb, &wbc); > + bdi_writeback_all(&wbc); > + wait_sb_inodes(&wbc); > return nr_to_write - wbc.nr_to_write; > } > EXPORT_SYMBOL(sync_inodes_sb); So to writeback or sync inodes in a single superblock, you effectively scan all the dirty inodes in the system just to find out which ones are on your superblock? I don't think that's very efficient. > diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h > index 928cd54..ac1d2ba 100644 > --- a/include/linux/backing-dev.h > +++ b/include/linux/backing-dev.h ... > +#define BDI_MAX_FLUSHERS 32 > + This isn't used anywhere... > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index f8341b6..2c287d9 100644 > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c ... > @@ -593,8 +582,15 @@ static void balance_dirty_pages(struct address_space *mapping) > if ((laptop_mode && pages_written) || > (!laptop_mode && (global_page_state(NR_FILE_DIRTY) > + global_page_state(NR_UNSTABLE_NFS) > - > background_thresh))) > - pdflush_operation(background_writeout, 0); > + > background_thresh))) { > + struct writeback_control wbc = { > + .bdi = bdi, > + .sync_mode = WB_SYNC_NONE, > + }; Shouldn't we set nr_pages here? I see that with your old code it wasn't needed because of that over_bground check but that will probably get changed. > + > + > + bdi_start_writeback(&wbc); > + } > } > Honza -- Jan Kara SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/