Re: [PATCH 1/9] mm: fix 100% CPU kswapd busyloop on unreclaimable nodes
From: Mel Gorman
Date: Thu Mar 09 2017 - 09:35:43 EST
On Tue, Mar 07, 2017 at 11:56:31AM -0500, Johannes Weiner wrote:
> On Tue, Mar 07, 2017 at 11:17:02AM +0100, Michal Hocko wrote:
> > On Mon 06-03-17 11:24:10, Johannes Weiner wrote:
> > > @@ -3271,7 +3271,8 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
> > > * Raise priority if scanning rate is too low or there was no
> > > * progress in reclaiming pages
> > > */
> > > - if (raise_priority || !sc.nr_reclaimed)
> > > + nr_reclaimed = sc.nr_reclaimed - nr_reclaimed;
> > > + if (raise_priority || !nr_reclaimed)
> > > sc.priority--;
> > > } while (sc.priority >= 1);
> > >
> > I would rather not play with the sc state here. From a quick look at
> > least
> > /*
> > * Fragmentation may mean that the system cannot be rebalanced for
> > * high-order allocations. If twice the allocation size has been
> > * reclaimed then recheck watermarks only at order-0 to prevent
> > * excessive reclaim. Assume that a process requested a high-order
> > * can direct reclaim/compact.
> > */
> > if (sc->order && sc->nr_reclaimed >= compact_gap(sc->order))
> > sc->order = 0;
> > does rely on the value. Wouldn't something like the following be safer?
> Well, what behavior is correct, though? This check looks like an
> argument *against* resetting sc.nr_reclaimed.
> If kswapd is woken up for a higher order, this check sets a reclaim
> cutoff beyond which it should give up on the order and balance for 0.
> That's on the scope of the kswapd invocation. Applying this threshold
> to the outcome of just the preceeding priority seems like a mistake.
> Mel? Vlastimil?
I cannot say which is definitely the correct behaviour. The current
behaviour is conservative due to the historical concerns about kswapd
reclaiming the world. The hazard as I see it is that resetting it *may*
lead to more aggressive reclaim for high-order allocations. That may be a
welcome outcome to some that really want high-order pages and be unwelcome
to others that prefer pages to remain resident.
However, in this case it's a tight window and problems would be tricky to
detect. THP allocations won't trigger the behaviour and with vmalloc'd
stack, I'd expect that only SLUB-intensive workloads using high-order
pages would trigger any adverse behaviour. While I'm mildly concerned, I
would be a little surprised if it actually caused runaway reclaim.