On Mon, Mar 14, 2011 at 1:23 PM, Vivek Goyal [off-list ref] wrote:

On Mon, Mar 14, 2011 at 11:29:17AM -0700, Greg Thelen wrote:

[..]

quoted

quoted

We could just crawl the memcg's page LRU and bring things under control
that way, couldn't we?  That would fix it.  What were the reasons for
not doing this?

My rational for pursuing bdi writeback was I/O locality.  I have heard that
per-page I/O has bad locality.  Per inode bdi-style writeback should have better
locality.

My hunch is the best solution is a hybrid which uses a) bdi writeback with a
target memcg filter and b) using the memcg lru as a fallback to identify the bdi
that needed writeback.  I think the part a) memcg filtering is likely something
like:
 http://marc.info/?l=linux-kernel&m=129910424431837

The part b) bdi selection should not be too hard assuming that page-to-mapping
locking is doable.

Greg,

IIUC, option b) seems to be going through pages of particular memcg and
mapping page to inode and start writeback on particular inode?

Yes.

If yes, this might be reasonably good. In the case when cgroups are not
sharing inodes then it automatically maps one inode to one cgroup and
once cgroup is over limit, it starts writebacks of its own inode.

In case inode is shared, then we get the case of one cgroup writting
back the pages of other cgroup. Well I guess that also can be handeled
by flusher thread where a bunch or group of pages can be compared with
the cgroup passed in writeback structure. I guess that might hurt us
more than benefit us.

Agreed.  For now just writing the entire inode is probably fine.

IIUC how option b) works then we don't even need option a) where an N level
deep cache is maintained?

Originally I was thinking that bdi-wide writeback with memcg filter
was a good idea.  But this may be unnecessarily complex.  Now I am
agreeing with you that option (a) may not be needed.  Memcg could
queue per-inode writeback using the memcg lru to locate inodes
(lru->page->inode) with something like this in
[mem_cgroup_]balance_dirty_pages():

  while (memcg_usage() >= memcg_fg_limit) {
    inode = memcg_dirty_inode(cg);  /* scan lru for a dirty page, then
grab mapping & inode */
    sync_inode(inode, &wbc);
  }

  if (memcg_usage() >= memcg_bg_limit) {
    queue per-memcg bg flush work item
  }

Does this look sensible?

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