Re: [PATCH v6 0/9] memcg: per cgroup dirty page accounting

[Jan Kara <jack@suse.cz>]

On Thu 17-03-11 13:12:19, Vivek Goyal wrote:
> On Thu, Mar 17, 2011 at 03:46:41PM +0100, Jan Kara wrote:
> [..]
> > > - bdi writeback: will revert some of the mmotm memcg dirty limit changes to
> > >   fs-writeback.c so that wb_do_writeback() will return to checking
> > >   wb_check_background_flush() to check background limits and being
> > > interruptible if
> > >   sync flush occurs.  wb_check_background_flush() will check the global
> > >   memcg_over_bg_limit list for memcg that are over their dirty limit.
> > >   wb_writeback() will either (I am not sure):
> > >   a) scan memcg's bdi_memcg list of inodes (only some of them are dirty)
> > >   b) scan bdi dirty inode list (only some of them in memcg) using
> > >      inode_in_memcg() to identify inodes to write.  inode_in_memcg(inode,memcg),
> > >      would walk memcg- -> memcg_bdi -> memcg_mapping to determine if the memcg
> > >      is caching pages from the inode.
> > Hmm, both has its problems. With a) we could queue all the dirty inodes
> > from the memcg for writeback but then we'd essentially write all dirty data
> > for a memcg, not only enough data to get below bg limit. And if we started
> > skipping inodes when memcg(s) inode belongs to get below bg limit, we'd
> > risk copying inodes there and back without reason, cases where some inodes
> > never get written because they always end up skipped etc. Also the question
> > whether some of the memcgs inode belongs to is still over limit is the
> > hardest part of solution b) so we wouldn't help ourselves much.
> 
> May be I am missing something but can't we just start traversing
> through list of memcg_over_bg_list and take option a) to traverse
> through list of inodes and write them till we are below limit of
> that group. We of course skip inodes which are not dirty.
> 
> This is assuming that root group is also part of that list so that
> inodes in root group do not starve writeback.
> 
> We still continue to have all the inodes on bdi wb structure and
> memcg will just give us pointers to those inodes. So for background
> write, instead of going serially through dirty inodes list, we
> will first pick the cgroup to write and then inode to write. As
> we will be doing round robin among cgroup list, it will make sure
> that none of the cgroups (including root) as well as inode are not
> starved.
  I was considering this as well and didn't quite like it but on a second
thought it need not be that bad. If we wrote MAX_WRITEBACK_PAGES from one
memcg, then switched to another one while keeping pointers to per-memcg inode
list (for the time when we return to this memcg), it could work just fine.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR