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

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Mar 14, 2011 at 07:41:13PM -0700, Greg Thelen wrote:
> On Mon, Mar 14, 2011 at 1:23 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> > On Mon, Mar 14, 2011 at 11:29:17AM -0700, Greg Thelen wrote:
> >
> > [..]
> >> > 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);
>   }

Is it possible to pass mem_cgroup in writeback_control structure or in
work structure which in turn will be set in writeback_control.  And
modify writeback_inodes_wb() which will look that ->mem_cgroup is
set. So instead of calling queue_io() it can call memcg_queue_io()
and then memory cgroup can look at lru list and take its own decision
on which inodes needs to be pushed for IO?

Thanks
Vivek