Re: [PATCH next 00/12] mm: replace struct mem_cgroup_zone with struct lruvec

[Hugh Dickins <hughd@google.com>]

On Fri, 27 Apr 2012, Konstantin Khlebnikov wrote:
> Andrew Morton wrote:
> > On Thu, 26 Apr 2012 11:53:44 +0400
> > Konstantin Khlebnikov<khlebnikov@openvz.org>  wrote:
> > 
> > > This patchset depends on Johannes Weiner's patch
> > > "mm: memcg: count pte references from every member of the reclaimed
> > > hierarchy".
> > > 
> > > bloat-o-meter delta for patches 2..12
> > > 
> > > add/remove: 6/6 grow/shrink: 6/14 up/down: 4414/-4625 (-211)
> > 
> > That's the sole effect and intent of the patchset?  To save 211 bytes?

I am surprised it's not more: it feels like more.

> 
> This is almost last bunch of cleanups for lru_lock splitting,
> code reducing is only nice side-effect.
> Also this patchset removes many redundant lruvec relookups.
> 
> Now mostly all page-to-lruvec translations are located at the same level
> as zone->lru_lock locking. So lru-lock splitting patchset can something like
> this:
> 
> -zone = page_zone(page)
> -spin_lock_irq(&zone->lru_lock)
> -lruvec = mem_cgroup_page_lruvec(page)
> +lruvec = lock_page_lruvec_irq(page)
> 
> > 
> > > ...
> > > 
> > >   include/linux/memcontrol.h |   16 +--
> > >   include/linux/mmzone.h     |   14 ++
> > >   mm/memcontrol.c            |   33 +++--
> > >   mm/mmzone.c                |   14 ++
> > >   mm/page_alloc.c            |    8 -
> > >   mm/vmscan.c                |  277
> > > ++++++++++++++++++++------------------------
> > >   6 files changed, 177 insertions(+), 185 deletions(-)
> > 
> > If so, I'm not sure that it is worth the risk and effort?

I'm pretty sure that it is worth the effort, and see very little risk.

It's close to my "[PATCH 3/10] mm/memcg: add zone pointer into lruvec"
posted 20 Feb (after Konstantin posted his set a few days earlier),
which Kamezawa-san Acked with "I like this cleanup".  But this goes
a little further (e.g. 01/12 saving an arg by moving priority into sc,
that's nice; and v2 05/12 removing update_isolated_counts(), great).

Konstantin and I came independently to this simplification, or
generalization, from zone to lruvec: we're confident that it is the
right direction, that it's a good basis for further work.  Certainly
neither of us have yet posted numbers to justify per-memcg per-zone
locking (and I expect split zone locking to need more justification
than it's had); but we both think these patches are a worthwhile
cleanup on their own.

I don't think it was particularly useful to split this into all of
12 pieces!  But never mind, that's a trivial detail, not worth undoing.
There's a few by-the-by bits and pieces I liked in my version that are
not here, but nothing important: if I care enough, I can always send a
little cleanup afterwards.

The only change I'd ask for is in the commit comment on 02/12: it
puzzlingly says "page_zone()" where it means to say "lruvec_zone()".
I think if I'd been doing 04/12, I'd have resented passing "zone" to
shrink_page_list(), would have deleted its VM_BUG_ON, and used a
page_zone() for ZONE_CONGESTED: but that's just me being mean.

I've gone through and compared the result of these 12 against my own
tree updated to next-20120427.  We come out much the same: the only
divergence which worried me was that my mem_cgroup_zone_lruvec() says
	IF (!memcg || mem_cgroup_disabled())
		return &zone->lruvec;
and although I'm sure I had a reason for adding that "!memcg || ",
I cannot now see why.  Maybe it was for some intermediate use that went
away (but I mention it in the hope that Konstantin will double check).

To each one of the 12 (with lruvec_zone in 02/12, and v2 of 05/12):
Acked-by: Hugh Dickins <hughd@google.com>

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