linux-kernel - Re: [RFC 0/8] Cpuset aware writeback

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Date:	Tue, 16 Jan 2007 23:00:34 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Christoph Lameter <clameter@....com>
Cc:	menage@...gle.com, linux-kernel@...r.kernel.org,
	nickpiggin@...oo.com.au, linux-mm@...ck.org, ak@...e.de,
	pj@....com, dgc@....com
Subject: Re: [RFC 0/8] Cpuset aware writeback

> On Tue, 16 Jan 2007 22:27:36 -0800 (PST) Christoph Lameter <clameter@....com> wrote:
> On Tue, 16 Jan 2007, Andrew Morton wrote:
> 
> > > Yes this is the result of the hierachical nature of cpusets which already 
> > > causes issues with the scheduler. It is rather typical that cpusets are 
> > > used to partition the memory and cpus. Overlappig cpusets seem to have 
> > > mainly an administrative function. Paul?
> > 
> > The typical usage scenarios don't matter a lot: the examples I gave show
> > that the core problem remains unsolved.  People can still hit the bug.
> 
> I agree the overlap issue is a problem and I hope it can be addressed 
> somehow for the rare cases in which such nesting takes place.
> 
> One easy solution may be to check the dirty ratio before engaging in 
> reclaim. If the dirty ratio is sufficiently high then trigger writeout via 
> pdflush (we already wakeup pdflush while scanning and you already noted 
> that pdflush writeout is not occurring within the context of the current 
> cpuset) and pass over any dirty pages during LRU scans until some pages 
> have been cleaned up.
> 
> This means we allow allocation of additional kernel memory outside of the 
> cpuset while triggering writeout of inodes that have pages on the nodes 
> of the cpuset. The memory directly used by the application is still 
> limited. Just the temporary information needed for writeback is allocated 
> outside.

Gad.  None of that should be necessary.

> Well sounds somehow still like a hack. Any other ideas out there?

Do what blockdevs do: limit the number of in-flight requests (Peter's
recent patch seems to be doing that for us) (perhaps only when PF_MEMALLOC
is in effect, to keep Trond happy) and implement a mempool for the NFS
request critical store.  Additionally:

- we might need to twiddle the NFS gfp_flags so it doesn't call the
  oom-killer on failure: just return NULL.

- consider going off-cpuset for critical allocations.  It's better than
  going oom.  A suitable implementation might be to ignore the caller's
  cpuset if PF_MEMALLOC.  Maybe put a WARN_ON_ONCE in there: we prefer that
  it not happen and we want to know when it does.

btw, regarding the per-address_space node mask: I think we should free it
when the inode is clean (!mapping_tagged(PAGECACHE_TAG_DIRTY)).  Chances
are, the inode will be dirty for 30 seconds and in-core for hours.  We
might as well steal its nodemask storage and give it to the next file which
gets written to.  A suitable place to do all this is in
__mark_inode_dirty(I_DIRTY_PAGES), using inode_lock to protect
address_space.dirty_page_nodemask.
-
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