On Thu, Jun 27, 2013 at 5:54 PM, Dave Chinner <[email protected]> wrote: > On Thu, Jun 27, 2013 at 04:54:53PM -1000, Linus Torvalds wrote: >> >> So what made it all start happening now? I don't recall us having had >> these kinds of issues before.. > > Not sure - it's a sudden surprise for me, too. Then again, I haven't > been looking at sync from a performance or lock contention point of > view any time recently. The algorithm that wait_sb_inodes() is > effectively unchanged since at least 2009, so it's probably a case > of it having been protected from contention by some external factor > we've fixed/removed recently. Perhaps the bdi-flusher thread > replacement in -rc1 has changed the timing sufficiently that it no > longer serialises concurrent sync calls as much.... > > However, the inode_sb_list_lock is known to be a badly contended > lock from a create/unlink fastpath for XFS, so it's not like this sort > of thing is completely unexpected.
That whole inode_sb_list_lock seems moronic. Why isn't it a per-sb
one? No, that won't fix all problems, but it might at least help a
*bit*.
Also, looking some more now at that wait_sb_inodes logic, I have to
say that if the problem is primarily the inode->i_lock, then that's
just crazy. We normally shouldn't even *need* that lock, since we
could do a totally unlocked iget() as long as the count is non-zero.
And no, I don't think really need the i_lock for checking
"mapping->nrpages == 0" or the magical "inode is being freed" bits
either. Or at least we could easily do some of this optimistically for
the common cases.
So instead of doing
struct address_space *mapping = inode->i_mapping;
spin_lock(&inode->i_lock);
if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
(mapping->nrpages == 0)) {
spin_unlock(&inode->i_lock);
continue;
}
__iget(inode);
spin_unlock(&inode->i_lock);
I really think we could do that without getting the inode lock at
*all* in the common case.
I'm attaching a pretty trivial patch, which may obviously be trivially
totally flawed. I have not tested this in any way, but half the new
lines are comments about why it's doing what it is doing. And I
really think that it should make the "actually take the inode lock" be
something quite rare.
And quite frankly, I'd much rather get *rid* of crazy i_lock accesses,
than try to be clever and use a whole different list at this point.
Not that I disagree that it wouldn't be much nicer to use a separate
list in the long run, but for a short-term solution I'd much rather
keep the old logic and just tweak it to be much more usable..
Hmm? Al? Jan? Comments?
Linus
patch.diff
Description: Binary data
