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SubjectRe: [PATCH] mm: stop kswapd's infinite loop at high order allocation
On Wed, Dec 31, 2008 at 12:16:47PM +0100, Nick Piggin wrote:
> On Wed, Dec 31, 2008 at 11:06:19AM +0000, Mel Gorman wrote:
> > On Wed, Dec 31, 2008 at 02:32:33AM +0100, Nick Piggin wrote:
> > > On Tue, Dec 30, 2008 at 06:59:19PM +0000, Mel Gorman wrote:
> > > > On Tue, Dec 30, 2008 at 07:55:47PM +0900, KOSAKI Motohiro wrote:
> > > > kswapd gets a sc.order when it is known there is a process trying to get
> > > > high-order pages so it can reclaim at that order in an attempt to prevent
> > > > future direct reclaim at a high-order. Your patch does not appear to depend on
> > > > GFP_KERNEL at all so I found the comment misleading. Furthermore, asking it to
> > > > loop again at order-0 means it may scan and reclaim more memory unnecessarily
> > > > seeing as all_zones_ok was calculated based on a high-order value, not order-0.
> > >
> > > It shouldn't, because it should check all that.
> > >
> >
> > Ok, with KOSAKI's patch we
> >
> > 1. Set order to 0 (and stop kswapd doing what it was asked to do)
> > 2. goto loop_again
> > 3. nr_reclaimed gets set to 0 (meaning we lose that value, but no biggie
> > as it doesn't get used by the caller anyway)
> > 4. Reset all priorities
> > 5. Do something like the following
> >
> > for (priority = DEF_PRIORITY; priority >= 0; priority--) {
> > ...
> > all_zones_ok = 1;
> > for (i = pgdat->nr_zones - 1; i >= 0; i--) {
> > ...
> > if (inactive_anon_is_low(zone)) {
> > shrink_active_list(SWAP_CLUSTER_MAX, zone,
> > &sc, priority, 0);
> > }
> >
> > if (!zone_watermark_ok(zone, order, zone->pages_high,
> > 0, 0)) {
> > end_zone = i;
> > break;
> > }
> > }
> > }
> >
> > So, by looping around, we could end up shrinking the active list again
> > before we recheck the zone watermarks depending on the size of the
> > inactive lists.
>
> If this is a problem, it is a problem with that code, because kswapd
> can be woken up for any zone at any time anyway.
>
>
> > > > cond_resched();
> > > >
> > > > try_to_freeze();
> > > >
> > > > goto loop_again;
> > > > }
> > > >
> > > > I used PAGE_ALLOC_COSTLY_ORDER instead of sc.order == 0 because we are
> > > > expected to support allocations up to that order in a fairly reliable fashion.
> > >
> > > I actually think it's better to do it for all orders, because that
> > > constant is more or less arbitrary.
> >
> > i.e.
> >
> > if (!all_zones_ok && sc.order == 0) {
> >
> > ? or something else
>
> Well, I jus tdon't see what's wrong with the original patch.
>

I've more or less convinced myself it's ok as any anomolies I spotted have
either been described as intentional behaviour or is arguably correct. A
fixed up (or deleted - misleading comments suck) comment and I'm happy.

>
> > What I did miss was that we have
> >
> > if (nr_reclaimed >= SWAP_CLUSTER_MAX)
> > break;
> >
> > so with my patch, kswapd is bailing out early without trying to reclaim for
> > high-orders that hard. That was not what I intended as it means we only ever
> > really rebalance the full system for order-0 pages and for everything else we
> > do relatively light scanning. The impact is that high-order users will direct
> > reclaim rather than depending on kswapd scanning very heavily. Arguably,
> > this is a good thing.
> >
> > However, it also means that KOSAKI's and my patches only differs in that mine
> > bails early and KOSAKI rechecks everything at order-0, possibly reclaiming
> > more. If the comment was not so misleading, I'd have been a lot happier.
>
> Rechecking everything is fine by me; order-0 is going to be the most
> common and most important. If higher order allocations sometimes have
> to enter direct reclaim or kick off kswapd again, it isn't a big deal.
>

Grand so. Initially it looked like accidental rather than intentional
behaviour but after thinking about it some more, it should be ok.

>
> > > IOW, I don't see a big downside, and there is a real upside.
> > >
> > > I think the patch is good.
> > >
> >
> > Which one, KOSAKI's or my one?
> >
> > Here is my one again which bails out for any high-order allocation after
> > just light scanning.
> >
> > ====
> >
> > >From 0e09fe002d8e9956de227b880ef8458842b71ca9 Mon Sep 17 00:00:00 2001
> > From: Mel Gorman <mel@csn.ul.ie>
> > Date: Tue, 30 Dec 2008 18:53:23 +0000
> > Subject: [PATCH] mm: stop kswapd's infinite loop at high order allocation
> >
> > Wassim Dagash reported the following (editted) kswapd infinite loop problem.
> >
> > kswapd runs in some infinite loop trying to swap until order 10 of zone
> > highmem is OK.... kswapd will continue to try to balance order 10 of zone
> > highmem forever (or until someone release a very large chunk of highmem).
> >
> > For costly high-order allocations, the system may never be balanced due to
> > fragmentation but kswapd should not infinitely loop as a result. The
> > following patch lets kswapd stop reclaiming in the event it cannot
> > balance zones and the order is high-order.
>
> This one bails out if it was a higher order reclaim, but there is still
> an order-0 shortage. I prefer to run the loop again at order==0 to avoid
> that condition. A higher kswapd reclaim order shouldn't weaken kswapd
> postcondition for order-0 memory.
>
> >
> > Reported-by: wassim dagash <wassim.dagash@gmail.com>
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> >
> > ---
> > mm/vmscan.c | 11 ++++++++++-
> > 1 files changed, 10 insertions(+), 1 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 62e7f62..7b0f412 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1867,7 +1867,16 @@ out:
> >
> > zone->prev_priority = temp_priority[i];
> > }
> > - if (!all_zones_ok) {
> > +
> > + /*
> > + * If zones are still not balanced, loop again and continue attempting
> > + * to rebalance the system. For high-order allocations, fragmentation
> > + * can prevent the zones being rebalanced no matter how hard kswapd
> > + * works, particularly on systems with little or no swap. For
> > + * high-orders, just give up and assume interested processes will
> > + * either direct reclaim or wake up kswapd again as necessary.
> > + */
> > + if (!all_zones_ok && sc.order == 0) {
> > cond_resched();
> >
> > try_to_freeze();
>
> --
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>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab


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