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Message-ID: <20090520113234.GT11363@kernel.dk>
Date: Wed, 20 May 2009 13:32:34 +0200
From: Jens Axboe <jens.axboe@...cle.com>
To: Jan Kara <jack@...e.cz>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
chris.mason@...cle.com, david@...morbit.com, hch@...radead.org,
akpm@...ux-foundation.org, yanmin_zhang@...ux.intel.com
Subject: Re: [PATCH 02/11] writeback: switch to per-bdi threads for
flushing data
On Wed, May 20 2009, Jan Kara wrote:
> Hi Jens,
>
> a few comments here. Mainly, I still don't think the sys_sync() is
> working right - see comments below.
Thanks! I took the liberty of killing some of the code in between, to
make it easier to see.
> > +void bdi_writeback_all(struct super_block *sb, long nr_pages)
> > +{
> > + struct backing_dev_info *bdi;
> > +
> > + rcu_read_lock();
> > +
> > +restart:
> > + list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
> Isn't the RCU list here a bit overengineering? AFAICS we use the list
> only here and if I'm grepping right, generic_sync_sb_inodes() is currently
> only used for data integrity sync (after your patches) from fs-writeback.c
> and by UBIFS to do equivalent of writeback_inodes(). So simple spinlock
> guarding the list should be just fine. Or am I missing something?
Sure, we could. But it's really not that much of a difference,
implementation wise.
> > @@ -591,13 +711,10 @@ static void generic_sync_bdi_inodes(struct backing_dev_info *bdi,
> > void generic_sync_sb_inodes(struct super_block *sb,
> > struct writeback_control *wbc)
> > {
> > - const int is_blkdev_sb = sb_is_blkdev_sb(sb);
> > - struct backing_dev_info *bdi;
> > -
> > - rcu_read_lock();
> > - list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
> > - generic_sync_bdi_inodes(bdi, wbc, sb, is_blkdev_sb);
> > - rcu_read_unlock();
> > + if (wbc->bdi)
> > + bdi_start_writeback(wbc->bdi, sb, 0);
> > + else
> > + bdi_writeback_all(sb, 0);
> It does not work like this. The way you call writeback here, you never
> endup calling __writeback_single_inode() with WB_SYNC_ALL set in wbc (your
> writeback routines always call inode writeback with WB_SYNC_NONE). And
> that is required for proper data integrity sync... So you have to somehow
> propagate this down to the writeback thread.
Good point, we need to pass down sync mode too. Not a big problem, we
can just add that to bdi_work as well.
> Alternatively, what probably makes a lot of sence, is to separate data
> integrity sync path from just data writeback. In the first case we care
> more about correctness, in the second case we care more about performance
> and overall throughput.
Yep agree, that would clean it up as well. I'll include that in the next
revision, I think I'll post it on friday.
> BTW your patch also significantly changes one thing: With your patch data
> integrity sync is done by flusher threads while previously is was done from
> the context of the thread calling sync(). I'm undecided whether it is a
> good or bad thing but it definitely deserves a comment in the changelog.
I'll look at the implications of this again, perhaps it'll be better to
just switch it back for now.
> > +static int bdi_forker_task(void *ptr)
> > +{
> > + struct backing_dev_info *bdi, *me = ptr;
> > +
> > + for (;;) {
> > + DEFINE_WAIT(wait);
> > +
> > + /*
> > + * Should never trigger on the default bdi
> > + */
> > + WARN_ON(bdi_has_dirty_io(me));
> > +
> > + prepare_to_wait(&me->wait, &wait, TASK_INTERRUPTIBLE);
> > + smp_mb();
> Wouldn't the code look simpler like:
> spin_lock_bh(&bdi_lock);
> if (list_empty(&bdi_pending_list)) {
> spin_unlock_bh(&bdi_lock);
> schedule();
> } else {
> bdi = list_entry(bdi_pending_list.next,
> struct backing_dev_info, bdi_list);
> list_del_init(&bdi->bdi_list);
> spin_unlock_bh(&bdi_lock);
> if (bdi->task)
> continue;
> ... do work ...
> }
Not a bad suggestion, I'll fiddle it around a bit.
>
> > + if (list_empty(&bdi_pending_list))
> > + schedule();
> > + else {
> > +repeat:
> > + bdi = NULL;
> > +
> > + spin_lock_bh(&bdi_lock);
> > + if (!list_empty(&bdi_pending_list)) {
> > + bdi = list_entry(bdi_pending_list.next,
> > + struct backing_dev_info,
> > + bdi_list);
> > + list_del_init(&bdi->bdi_list);
> > + }
> > + spin_unlock_bh(&bdi_lock);
> > +
> > + /*
> > + * If no bdi or bdi already got setup, continue
> > + */
> > + if (!bdi || bdi->task)
> > + continue;
> > +
> > + bdi->task = kthread_run(bdi_start_fn, bdi, "bdi-%s",
> > + dev_name(bdi->dev));
> > + /*
> > + * If task creation fails, then readd the bdi to
> > + * the pending list and force writeout of the bdi
> > + * from this forker thread. That will free some memory
> > + * and we can try again.
> > + */
> > + if (!bdi->task) {
> > + struct writeback_control wbc = {
> > + .bdi = bdi,
> > + .sync_mode = WB_SYNC_NONE,
> > + .older_than_this = NULL,
> > + .range_cyclic = 1,
> > + };
> > +
> > + /*
> > + * Add this 'bdi' to the back, so we get
> > + * a chance to flush other bdi's to free
> > + * memory.
> > + */
> > + spin_lock_bh(&bdi_lock);
> > + list_add_tail(&bdi->bdi_list,
> > + &bdi_pending_list);
> > + spin_unlock_bh(&bdi_lock);
> > +
> > + wbc.nr_to_write = 1024;
> > + generic_sync_bdi_inodes(NULL, &wbc);
> > + goto repeat;
> > + }
> > + }
> > +
> > + finish_wait(&me->wait, &wait);
> > + }
> > +
> > + return 0;
Thanks for your review Jan, always helpful!
--
Jens Axboe
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