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SubjectRe: [PATCH 6/6] writeback: merge for_kupdate and !for_kupdate cases
On Mon, Jul 12, 2010 at 10:08:42AM +0800, Dave Chinner wrote:
> On Sun, Jul 11, 2010 at 10:07:02AM +0800, Wu Fengguang wrote:
> > Unify the logic for kupdate and non-kupdate cases.
> > There won't be starvation because the inodes requeued into b_more_io
> > will later be spliced _after_ the remaining inodes in b_io, hence won't
> > stand in the way of other inodes in the next run.
> >
> > It avoids unnecessary redirty_tail() calls, hence the update of
> > i_dirtied_when. The timestamp update is undesirable because it could
> > later delay the inode's periodic writeback, or exclude the inode from
> > the data integrity sync operation (which will check timestamp to avoid
> > extra work and livelock).
> >
> > CC: Dave Chinner <david@fromorbit.com>
> > Cc: Martin Bligh <mbligh@google.com>
> > Cc: Michael Rubin <mrubin@google.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> > fs/fs-writeback.c | 39 ++++++---------------------------------
> > 1 file changed, 6 insertions(+), 33 deletions(-)
> >
> > --- linux-next.orig/fs/fs-writeback.c 2010-07-11 09:13:32.000000000 +0800
> > +++ linux-next/fs/fs-writeback.c 2010-07-11 09:13:36.000000000 +0800
> > @@ -373,45 +373,18 @@ writeback_single_inode(struct inode *ino
> > if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
> > /*
> > * We didn't write back all the pages. nfs_writepages()
> > - * sometimes bales out without doing anything. Redirty
> > - * the inode; Move it from b_io onto b_more_io/b_dirty.
> > + * sometimes bales out without doing anything.
> > */
> > - /*
> > - * akpm: if the caller was the kupdate function we put
> > - * this inode at the head of b_dirty so it gets first
> > - * consideration. Otherwise, move it to the tail, for
> > - * the reasons described there. I'm not really sure
> > - * how much sense this makes. Presumably I had a good
> > - * reasons for doing it this way, and I'd rather not
> > - * muck with it at present.
> > - */
> > - if (wbc->for_kupdate) {
> > + inode->i_state |= I_DIRTY_PAGES;
> > + if (wbc->nr_to_write <= 0) {
> > /*
> > - * For the kupdate function we move the inode
> > - * to b_more_io so it will get more writeout as
> > - * soon as the queue becomes uncongested.
> > + * slice used up: queue for next turn
> > */
> > - inode->i_state |= I_DIRTY_PAGES;
> > - if (wbc->nr_to_write <= 0) {
> > - /*
> > - * slice used up: queue for next turn
> > - */
> > - requeue_io(inode);
> > - } else {
> > - /*
> > - * somehow blocked: retry later
> > - */
> > - redirty_tail(inode);
> > - }
> > + requeue_io(inode);
> > } else {
> > /*
> > - * Otherwise fully redirty the inode so that
> > - * other inodes on this superblock will get some
> > - * writeout. Otherwise heavy writing to one
> > - * file would indefinitely suspend writeout of
> > - * all the other files.
> > + * somehow blocked: retry later
> > */
> > - inode->i_state |= I_DIRTY_PAGES;
> > redirty_tail(inode);
> > }
>
> This means that congestion will always trigger redirty_tail(). Is
> that really what we want for that case?

This patch actually converts some redirty_tail() cases to use
requeue_io(), so are reducing the use of redirty_tail(). Also
recent kernels are blocked _inside_ get_request() on congestion
instead of returning to writeback_single_inode() on congestion.
So the "somehow blocked" comment for redirty_tail() no longer includes
the congestion case.

> Also, I'd prefer that the
> comments remain somewhat more descriptive of the circumstances that
> we are operating under. Comments like "retry later to avoid blocking
> writeback of other inodes" is far, far better than "retry later"
> because it has "why" component that explains the reason for the
> logic. You may remember why, but I sure won't in a few months time....

Ah yes the comment is too simple. However the redirty_tail() is not to
avoid blocking writeback of other inodes, but to avoid eating 100% CPU
on busy retrying a dirty inode/page that cannot perform writeback for
a while. (In theory redirty_tail() can still busy retry though, when
there is only one single dirty inode.) So how about

/*
* somehow blocked: avoid busy retrying
*/
Thanks,
Fengguang


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